feat: Add tier-based vehicle limit enforcement (refs #23)

Backend:
- Add VEHICLE_LIMITS configuration to feature-tiers.ts
- Add getVehicleLimit, canAddVehicle helper functions
- Implement transaction-based limit check with FOR UPDATE locking
- Add VehicleLimitExceededError and 403 TIER_REQUIRED response
- Add countByUserId to VehiclesRepository
- Add comprehensive tests for all limit logic

Frontend:
- Add getResourceLimit, isAtResourceLimit to useTierAccess hook
- Create VehicleLimitDialog component with mobile/desktop modes
- Add useVehicleLimitCheck shared hook for limit state
- Update VehiclesPage with limit checks and lock icon
- Update VehiclesMobileScreen with limit checks
- Add tests for VehicleLimitDialog

Implements vehicle limits per tier (Free: 2, Pro: 5, Enterprise: unlimited)
with race condition prevention and consistent UX across mobile/desktop.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/core/hooks/useTierAccess.ts

@@ -33,6 +33,15 @@ const TIER_LEVELS: Record<SubscriptionTier, number> = {
   enterprise: 2,
 };
 
+// Resource limits per tier (mirrors backend VEHICLE_LIMITS)
+const RESOURCE_LIMITS = {
+  vehicles: {
+    free: 2,
+    pro: 5,
+    enterprise: null, // unlimited
+  } as Record<SubscriptionTier, number | null>,
+};
+
 /**
  * Hook to check if user can access tier-gated features
  * Fetches user profile for tier and feature config from backend
@@ -100,11 +109,47 @@ export const useTierAccess = () => {
     };
   };
 
+  /**
+   * Get resource limit for current user's tier
+   * Resource-agnostic method for count-based limits (vehicles, documents, etc.)
+   *
+   * @param resourceType - Type of resource (e.g., 'vehicles')
+   * @returns Maximum allowed count, or null for unlimited
+   */
+  const getResourceLimit = (resourceType: keyof typeof RESOURCE_LIMITS): number | null => {
+    const limits = RESOURCE_LIMITS[resourceType];
+    if (!limits) {
+      return null; // Unknown resource type = unlimited
+    }
+    return limits[tier] ?? null;
+  };
+
+  /**
+   * Check if user is at or over their resource limit
+   * Resource-agnostic method for count-based limits (vehicles, documents, etc.)
+   *
+   * @param resourceType - Type of resource (e.g., 'vehicles')
+   * @param currentCount - Current number of resources user has
+   * @returns true if user is at or over limit, false if under limit or unlimited
+   */
+  const isAtResourceLimit = (
+    resourceType: keyof typeof RESOURCE_LIMITS,
+    currentCount: number
+  ): boolean => {
+    const limit = getResourceLimit(resourceType);
+    if (limit === null) {
+      return false; // Unlimited
+    }
+    return currentCount >= limit;
+  };
+
   return {
     tier,
     loading: profileQuery.isLoading || featureConfigQuery.isLoading,
     hasAccess,
     checkAccess,
+    getResourceLimit,
+    isAtResourceLimit,
   };
 };
 

frontend/src/features/vehicles/hooks/useVehicleLimitCheck.ts

@@ -0,0 +1,23 @@
+/**
+ * @ai-summary Hook for checking vehicle limit and managing limit dialog
+ * @ai-context Shared between desktop and mobile vehicle pages
+ */
+
+import { useState } from 'react';
+import { useTierAccess } from '../../../core/hooks/useTierAccess';
+
+export const useVehicleLimitCheck = (vehicleCount: number) => {
+  const { tier, isAtResourceLimit, getResourceLimit } = useTierAccess();
+  const [showLimitDialog, setShowLimitDialog] = useState(false);
+
+  const isAtLimit = isAtResourceLimit('vehicles', vehicleCount);
+  const limit = getResourceLimit('vehicles');
+
+  return {
+    isAtLimit,
+    limit,
+    tier,
+    showLimitDialog,
+    setShowLimitDialog,
+  };
+};

frontend/src/features/vehicles/mobile/VehiclesMobileScreen.tsx

@@ -6,10 +6,13 @@
 import React, { useTransition, useMemo } from 'react';
 import { Box, Typography, Grid, Button } from '@mui/material';
 import AddIcon from '@mui/icons-material/Add';
+import LockIcon from '@mui/icons-material/Lock';
 import { useVehicles } from '../hooks/useVehicles';
 import { useOptimisticVehicles } from '../hooks/useOptimisticVehicles';
 import { useVehicleSearch } from '../hooks/useVehicleTransitions';
+import { useVehicleLimitCheck } from '../hooks/useVehicleLimitCheck';
 import { MobileVehiclesSuspense } from '../../../components/SuspenseWrappers';
+import { VehicleLimitDialog } from '../../../shared-minimal/components';
 import { VehicleMobileCard } from './VehicleMobileCard';
 import { Vehicle } from '../types/vehicles.types';
 
@@ -56,6 +59,15 @@ export const VehiclesMobileScreen: React.FC<VehiclesMobileScreenProps> = ({
   // Enhanced search with transitions (auto-syncs when vehicles change)
   const { filteredVehicles } = useVehicleSearch(optimisticVehicles);
 
+  // Vehicle limit check
+  const {
+    isAtLimit,
+    limit,
+    tier,
+    showLimitDialog,
+    setShowLimitDialog,
+  } = useVehicleLimitCheck(safeVehicles.length);
+
   const handleVehicleSelect = (vehicle: Vehicle) => {
     // Use transition to avoid blocking UI during navigation
     startTransition(() => {
@@ -63,6 +75,14 @@ export const VehiclesMobileScreen: React.FC<VehiclesMobileScreenProps> = ({
     });
   };
 
+  const handleAddVehicleClick = () => {
+    if (isAtLimit) {
+      setShowLimitDialog(true);
+    } else {
+      onAddVehicle?.();
+    }
+  };
+
   if (isLoading) {
     return (
       <Box sx={{ pb: 10 }}>
@@ -92,8 +112,8 @@ export const VehiclesMobileScreen: React.FC<VehiclesMobileScreenProps> = ({
             <Button
               variant="contained"
               color="primary"
-              startIcon={<AddIcon />}
-              onClick={() => onAddVehicle?.()}
+              startIcon={isAtLimit ? <LockIcon /> : <AddIcon />}
+              onClick={handleAddVehicleClick}
               sx={{ minWidth: 160 }}
             >
               Add Vehicle
@@ -119,6 +139,15 @@ export const VehiclesMobileScreen: React.FC<VehiclesMobileScreenProps> = ({
             ))}
           </Grid>
         </Section>
+
+        {/* Vehicle Limit Dialog */}
+        <VehicleLimitDialog
+          open={showLimitDialog}
+          onClose={() => setShowLimitDialog(false)}
+          currentCount={safeVehicles.length}
+          limit={limit ?? 0}
+          currentTier={tier}
+        />
       </Box>
     </MobileVehiclesSuspense>
   );

frontend/src/features/vehicles/pages/VehiclesPage.tsx

@@ -6,14 +6,17 @@
 import React, { useState, useTransition, useMemo, useEffect } from 'react';
 import { Box, Typography, Grid, Button as MuiButton, TextField, IconButton } from '@mui/material';
 import AddIcon from '@mui/icons-material/Add';
+import LockIcon from '@mui/icons-material/Lock';
 import SearchIcon from '@mui/icons-material/Search';
 import ClearIcon from '@mui/icons-material/Clear';
 import { useVehicles } from '../hooks/useVehicles';
 import { useOptimisticVehicles } from '../hooks/useOptimisticVehicles';
 import { useVehicleSearch } from '../hooks/useVehicleTransitions';
+import { useVehicleLimitCheck } from '../hooks/useVehicleLimitCheck';
 import { VehicleCard } from '../components/VehicleCard';
 import { VehicleForm } from '../components/VehicleForm';
 import { Card } from '../../../shared-minimal/components/Card';
+import { VehicleLimitDialog } from '../../../shared-minimal/components';
 import { VehicleListSuspense, FormSuspense } from '../../../components/SuspenseWrappers';
 import { useAppStore } from '../../../core/store';
 import { useNavigate, useLocation } from 'react-router-dom';
@@ -53,6 +56,15 @@ export const VehiclesPage: React.FC = () => {
   const [showForm, setShowForm] = useState(false);
   const [stagedImageFile, setStagedImageFile] = useState<File | null>(null);
 
+  // Vehicle limit check
+  const {
+    isAtLimit,
+    limit,
+    tier,
+    showLimitDialog,
+    setShowLimitDialog,
+  } = useVehicleLimitCheck(safeVehicles.length);
+
   // Auto-show form if navigated with showAddForm state (from dashboard)
   useEffect(() => {
     const state = location.state as { showAddForm?: boolean } | null;
@@ -129,23 +141,31 @@ export const VehiclesPage: React.FC = () => {
     );
   }
 
+  const handleAddVehicleClick = () => {
+    if (isAtLimit) {
+      setShowLimitDialog(true);
+    } else {
+      startTransition(() => setShowForm(true));
+    }
+  };
+
   return (
     <VehicleListSuspense>
       <Box sx={{ py: 2 }}>
-        <Box sx={{ 
-          display: 'flex', 
-          justifyContent: 'space-between', 
-          alignItems: 'center', 
-          mb: 4 
+        <Box sx={{
+          display: 'flex',
+          justifyContent: 'space-between',
+          alignItems: 'center',
+          mb: 4
         }}>
           <Typography variant="h4" sx={{ fontWeight: 700, color: 'text.primary' }}>
             My Vehicles
           </Typography>
           {!showForm && (
-            <MuiButton 
-              variant="contained" 
-              startIcon={<AddIcon />}
-              onClick={() => startTransition(() => setShowForm(true))}
+            <MuiButton
+              variant="contained"
+              startIcon={isAtLimit ? <LockIcon /> : <AddIcon />}
+              onClick={handleAddVehicleClick}
               sx={{ borderRadius: '999px' }}
               disabled={isPending || isOptimisticPending}
             >
@@ -208,10 +228,10 @@ export const VehiclesPage: React.FC = () => {
               No vehicles added yet
             </Typography>
             {!showForm && (
-              <MuiButton 
-                variant="contained" 
-                startIcon={<AddIcon />}
-                onClick={() => startTransition(() => setShowForm(true))}
+              <MuiButton
+                variant="contained"
+                startIcon={isAtLimit ? <LockIcon /> : <AddIcon />}
+                onClick={handleAddVehicleClick}
                 sx={{ borderRadius: '999px' }}
                 disabled={isPending || isOptimisticPending}
               >
@@ -234,6 +254,15 @@ export const VehiclesPage: React.FC = () => {
           ))}
         </Grid>
       )}
+
+      {/* Vehicle Limit Dialog */}
+      <VehicleLimitDialog
+        open={showLimitDialog}
+        onClose={() => setShowLimitDialog(false)}
+        currentCount={safeVehicles.length}
+        limit={limit ?? 0}
+        currentTier={tier}
+      />
       </Box>
     </VehicleListSuspense>
   );

frontend/src/shared-minimal/components/VehicleLimitDialog.test.tsx

@@ -0,0 +1,225 @@
+/**
+ * @ai-summary Tests for VehicleLimitDialog component
+ * @ai-context Validates props, mobile/desktop modes, and user interactions
+ */
+
+import React from 'react';
+import { render, screen, fireEvent } from '@testing-library/react';
+import '@testing-library/jest-dom';
+import { VehicleLimitDialog } from './VehicleLimitDialog';
+
+// Mock MUI useMediaQuery to control mobile/desktop mode
+jest.mock('@mui/material', () => ({
+  ...jest.requireActual('@mui/material'),
+  useMediaQuery: jest.fn(),
+}));
+
+import { useMediaQuery } from '@mui/material';
+const mockedUseMediaQuery = useMediaQuery as jest.MockedFunction<typeof useMediaQuery>;
+
+describe('VehicleLimitDialog', () => {
+  const mockOnClose = jest.fn();
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    // Default to desktop mode
+    mockedUseMediaQuery.mockReturnValue(false);
+  });
+
+  describe('Dialog rendering', () => {
+    it('renders when open', () => {
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      expect(screen.getByText('Vehicle Limit Reached')).toBeInTheDocument();
+      expect(screen.getByText('Additional Vehicles')).toBeInTheDocument();
+    });
+
+    it('does not render when closed', () => {
+      render(
+        <VehicleLimitDialog
+          open={false}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      expect(screen.queryByText('Vehicle Limit Reached')).not.toBeInTheDocument();
+    });
+  });
+
+  describe('Props display', () => {
+    it('displays current count and limit', () => {
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      expect(screen.getByText('You have 2 of 2 vehicles')).toBeInTheDocument();
+    });
+
+    it('displays free tier upgrade prompt', () => {
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      expect(
+        screen.getByText(/Free tier is limited to 2 vehicles/)
+      ).toBeInTheDocument();
+    });
+
+    it('displays pro tier upgrade prompt', () => {
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={5}
+          limit={5}
+          currentTier="pro"
+        />
+      );
+
+      expect(
+        screen.getByText(/Pro tier is limited to 5 vehicles/)
+      ).toBeInTheDocument();
+    });
+
+    it('shows tier chips for free user', () => {
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      expect(screen.getByText('Free')).toBeInTheDocument();
+      expect(screen.getByText('Pro')).toBeInTheDocument();
+    });
+
+    it('shows tier chips for pro user', () => {
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={5}
+          limit={5}
+          currentTier="pro"
+        />
+      );
+
+      expect(screen.getByText('Pro')).toBeInTheDocument();
+      expect(screen.getByText('Enterprise')).toBeInTheDocument();
+    });
+  });
+
+  describe('User interactions', () => {
+    it('calls onClose when "Maybe Later" is clicked', () => {
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      fireEvent.click(screen.getByText('Maybe Later'));
+      expect(mockOnClose).toHaveBeenCalledTimes(1);
+    });
+
+    it('calls onClose when "Upgrade (Coming Soon)" is clicked', () => {
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      fireEvent.click(screen.getByText('Upgrade (Coming Soon)'));
+      expect(mockOnClose).toHaveBeenCalledTimes(1);
+    });
+  });
+
+  describe('Mobile responsiveness', () => {
+    it('renders fullscreen on mobile', () => {
+      mockedUseMediaQuery.mockReturnValue(true); // Mobile mode
+
+      const { container } = render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      // Check for fullScreen prop on Dialog (would be in DOM structure)
+      const dialog = container.querySelector('[role="dialog"]');
+      expect(dialog).toBeInTheDocument();
+    });
+
+    it('shows close button on mobile', () => {
+      mockedUseMediaQuery.mockReturnValue(true); // Mobile mode
+
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      const closeButton = screen.getByLabelText('close');
+      expect(closeButton).toBeInTheDocument();
+
+      fireEvent.click(closeButton);
+      expect(mockOnClose).toHaveBeenCalledTimes(1);
+    });
+
+    it('hides close button on desktop', () => {
+      mockedUseMediaQuery.mockReturnValue(false); // Desktop mode
+
+      render(
+        <VehicleLimitDialog
+          open={true}
+          onClose={mockOnClose}
+          currentCount={2}
+          limit={2}
+          currentTier="free"
+        />
+      );
+
+      expect(screen.queryByLabelText('close')).not.toBeInTheDocument();
+    });
+  });
+});

frontend/src/shared-minimal/components/VehicleLimitDialog.tsx

@@ -0,0 +1,213 @@
+/**
+ * @ai-summary Dialog shown when users reach their vehicle limit
+ * @ai-context Displays tier comparison and upgrade prompt for vehicle limits
+ */
+
+import React from 'react';
+import {
+  Dialog,
+  DialogTitle,
+  DialogContent,
+  DialogActions,
+  Button,
+  Typography,
+  Box,
+  Chip,
+  useMediaQuery,
+  useTheme,
+  IconButton,
+} from '@mui/material';
+import LockOutlinedIcon from '@mui/icons-material/LockOutlined';
+import CloseIcon from '@mui/icons-material/Close';
+import type { SubscriptionTier } from '../../features/settings/types/profile.types';
+
+interface VehicleLimitDialogProps {
+  open: boolean;
+  onClose: () => void;
+  currentCount: number;
+  limit: number;
+  currentTier: SubscriptionTier;
+}
+
+const tierDisplayNames: Record<SubscriptionTier, string> = {
+  free: 'Free',
+  pro: 'Pro',
+  enterprise: 'Enterprise',
+};
+
+const tierColors: Record<SubscriptionTier, 'default' | 'primary' | 'secondary'> = {
+  free: 'default',
+  pro: 'primary',
+  enterprise: 'secondary',
+};
+
+const getUpgradePrompt = (tier: SubscriptionTier): string => {
+  if (tier === 'free') {
+    return 'Free tier is limited to 2 vehicles. Upgrade to Pro for up to 5 vehicles, or Enterprise for unlimited.';
+  } else if (tier === 'pro') {
+    return 'Pro tier is limited to 5 vehicles. Upgrade to Enterprise for unlimited vehicles.';
+  }
+  return 'Upgrade to access additional vehicles.';
+};
+
+const getRequiredTier = (tier: SubscriptionTier): SubscriptionTier => {
+  if (tier === 'free') return 'pro';
+  if (tier === 'pro') return 'enterprise';
+  return 'pro';
+};
+
+export const VehicleLimitDialog: React.FC<VehicleLimitDialogProps> = ({
+  open,
+  onClose,
+  currentCount,
+  limit,
+  currentTier,
+}) => {
+  const theme = useTheme();
+  const isSmall = useMediaQuery(theme.breakpoints.down('sm'));
+  const requiredTier = getRequiredTier(currentTier);
+  const upgradePrompt = getUpgradePrompt(currentTier);
+
+  const handleUpgradeClick = () => {
+    // Navigate to upgrade page when Stripe integration is added
+    // For now, just close the dialog
+    onClose();
+  };
+
+  return (
+    <Dialog
+      open={open}
+      onClose={onClose}
+      fullScreen={isSmall}
+      maxWidth="sm"
+      fullWidth
+      PaperProps={{
+        sx: {
+          maxHeight: isSmall ? '100%' : '90vh',
+          m: isSmall ? 0 : 2,
+        },
+      }}
+    >
+      {isSmall && (
+        <IconButton
+          aria-label="close"
+          onClick={onClose}
+          sx={{
+            position: 'absolute',
+            right: 8,
+            top: 8,
+            color: (theme) => theme.palette.grey[500],
+          }}
+        >
+          <CloseIcon />
+        </IconButton>
+      )}
+
+      <DialogTitle sx={{ display: 'flex', alignItems: 'center', gap: 1 }}>
+        <LockOutlinedIcon color="action" />
+        Vehicle Limit Reached
+      </DialogTitle>
+
+      <DialogContent>
+        <Box sx={{ display: 'flex', flexDirection: 'column', gap: 3 }}>
+          {/* Current status */}
+          <Box>
+            <Typography variant="h6" gutterBottom>
+              Additional Vehicles
+            </Typography>
+            <Typography variant="body2" color="text.secondary">
+              {upgradePrompt}
+            </Typography>
+          </Box>
+
+          {/* Current count indicator */}
+          <Box
+            sx={{
+              display: 'flex',
+              alignItems: 'center',
+              justifyContent: 'center',
+              py: 2,
+              px: 3,
+              bgcolor: 'warning.light',
+              borderRadius: 1,
+              color: 'warning.contrastText',
+            }}
+          >
+            <Typography variant="body1" fontWeight="medium">
+              You have {currentCount} of {limit} vehicles
+            </Typography>
+          </Box>
+
+          {/* Tier comparison */}
+          <Box
+            sx={{
+              display: 'flex',
+              alignItems: 'center',
+              justifyContent: 'center',
+              gap: 2,
+              py: 2,
+              px: 3,
+              bgcolor: 'action.hover',
+              borderRadius: 1,
+            }}
+          >
+            <Box sx={{ textAlign: 'center' }}>
+              <Typography variant="caption" color="text.secondary" display="block">
+                Your Plan
+              </Typography>
+              <Chip
+                label={tierDisplayNames[currentTier]}
+                color={tierColors[currentTier]}
+                size="small"
+                sx={{ mt: 0.5 }}
+              />
+            </Box>
+
+            <Typography variant="h5" color="text.secondary">
+              →
+            </Typography>
+
+            <Box sx={{ textAlign: 'center' }}>
+              <Typography variant="caption" color="text.secondary" display="block">
+                Upgrade to
+              </Typography>
+              <Chip
+                label={tierDisplayNames[requiredTier]}
+                color={tierColors[requiredTier]}
+                size="small"
+                sx={{ mt: 0.5 }}
+              />
+            </Box>
+          </Box>
+
+          {/* Benefits preview */}
+          <Typography variant="body2" color="text.secondary">
+            Upgrade to unlock {requiredTier === 'enterprise' ? 'unlimited vehicles' : 'more vehicles'} and other premium capabilities.
+          </Typography>
+        </Box>
+      </DialogContent>
+
+      <DialogActions sx={{ px: 3, pb: 3, pt: 1, flexDirection: isSmall ? 'column' : 'row', gap: 1 }}>
+        <Button
+          onClick={onClose}
+          variant="outlined"
+          fullWidth={isSmall}
+          sx={{ order: isSmall ? 2 : 1 }}
+        >
+          Maybe Later
+        </Button>
+        <Button
+          onClick={handleUpgradeClick}
+          variant="contained"
+          color="primary"
+          fullWidth={isSmall}
+          sx={{ order: isSmall ? 1 : 2 }}
+        >
+          Upgrade (Coming Soon)
+        </Button>
+      </DialogActions>
+    </Dialog>
+  );
+};
+
+export default VehicleLimitDialog;

frontend/src/shared-minimal/components/index.ts

@@ -5,3 +5,4 @@
 export { Button } from './Button';
 export { Card } from './Card';
 export { UpgradeRequiredDialog } from './UpgradeRequiredDialog';
+export { VehicleLimitDialog } from './VehicleLimitDialog';