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/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,
+  };
+};