fix: filter locked vehicles after tier downgrade selection (refs #60)

- GET /api/vehicles now uses getUserVehiclesWithTierStatus() and filters
  out vehicles with tierStatus='locked' so only selected vehicles appear
  in the vehicle list
- GET /api/vehicles/:id now checks tier status and returns 403 TIER_REQUIRED
  if user tries to access a locked vehicle directly

This ensures that after a user selects 2 vehicles during downgrade to
free tier, only those 2 vehicles appear in the summary and details screens.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

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

@@ -28,7 +28,12 @@ export class VehiclesController {
   async getUserVehicles(request: FastifyRequest, reply: FastifyReply) {
     try {
       const userId = (request as any).user.sub;
-      const vehicles = await this.vehiclesService.getUserVehicles(userId);
+      // Use tier-aware method to filter out locked vehicles after downgrade
+      const vehiclesWithStatus = await this.vehiclesService.getUserVehiclesWithTierStatus(userId);
+      // Only return active vehicles (filter out locked ones)
+      const vehicles = vehiclesWithStatus
+        .filter(v => v.tierStatus === 'active')
+        .map(({ tierStatus, ...vehicle }) => vehicle);
 
       return reply.code(200).send(vehicles);
     } catch (error) {
@@ -108,6 +113,20 @@ export class VehiclesController {
       const userId = (request as any).user.sub;
       const { id } = request.params;
 
+      // Check tier status - block access to locked vehicles
+      const vehiclesWithStatus = await this.vehiclesService.getUserVehiclesWithTierStatus(userId);
+      const vehicleStatus = vehiclesWithStatus.find(v => v.id === id);
+      if (vehicleStatus && vehicleStatus.tierStatus === 'locked') {
+        return reply.code(403).send({
+          error: 'TIER_REQUIRED',
+          requiredTier: 'pro',
+          feature: 'vehicle.access',
+          featureName: 'Vehicle Access',
+          upgradePrompt: 'Upgrade to Pro to access all your vehicles',
+          message: 'This vehicle is not available on your current subscription tier'
+        });
+      }
+
       const vehicle = await this.vehiclesService.getVehicle(id, userId);
 
       return reply.code(200).send(vehicle);