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,
   };
 };