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>

backend/src/features/vehicles/api/vehicles.controller.ts

@@ -4,7 +4,7 @@
  */
 
 import { FastifyRequest, FastifyReply } from 'fastify';
-import { VehiclesService } from '../domain/vehicles.service';
+import { VehiclesService, VehicleLimitExceededError } from '../domain/vehicles.service';
 import { VehiclesRepository } from '../data/vehicles.repository';
 import { pool } from '../../../core/config/database';
 import { logger } from '../../../core/logging/logger';
@@ -21,7 +21,7 @@ export class VehiclesController {
 
   constructor() {
     const repository = new VehiclesRepository(pool);
-    this.vehiclesService = new VehiclesService(repository);
+    this.vehiclesService = new VehiclesService(repository, pool);
     this.nhtsaClient = new NHTSAClient(pool);
   }
 
@@ -62,25 +62,40 @@ export class VehiclesController {
 
       const userId = (request as any).user.sub;
       const vehicle = await this.vehiclesService.createVehicle(request.body, userId);
-      
+
       return reply.code(201).send(vehicle);
     } catch (error: any) {
       logger.error('Error creating vehicle', { error, userId: (request as any).user?.sub });
-      
+
+      if (error instanceof VehicleLimitExceededError) {
+        return reply.code(403).send({
+          error: 'TIER_REQUIRED',
+          requiredTier: error.tier === 'free' ? 'pro' : 'enterprise',
+          currentTier: error.tier,
+          feature: 'vehicle.addBeyondLimit',
+          featureName: 'Additional Vehicles',
+          upgradePrompt: error.upgradePrompt,
+          context: {
+            limit: error.limit,
+            count: error.currentCount
+          }
+        });
+      }
+
       if (error.message === 'Invalid VIN format') {
         return reply.code(400).send({
           error: 'Bad Request',
           message: error.message
         });
       }
-      
+
       if (error.message === 'Vehicle with this VIN already exists') {
         return reply.code(400).send({
           error: 'Bad Request',
           message: error.message
         });
       }
-      
+
       return reply.code(500).send({
         error: 'Internal server error',
         message: 'Failed to create vehicle'