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/core/config/tests/feature-tiers.test.ts

@@ -1,11 +1,15 @@
 import {
   TIER_LEVELS,
   FEATURE_TIERS,
+  VEHICLE_LIMITS,
   getTierLevel,
   canAccessFeature,
   getRequiredTier,
   getFeatureConfig,
   getAllFeatureConfigs,
+  getVehicleLimit,
+  canAddVehicle,
+  getVehicleLimitConfig,
 } from '../feature-tiers';
 
 describe('feature-tiers', () => {
@@ -101,4 +105,97 @@ describe('feature-tiers', () => {
       expect(FEATURE_TIERS['test' as keyof typeof FEATURE_TIERS]).toBeUndefined();
     });
   });
+
+  describe('VEHICLE_LIMITS', () => {
+    it('defines correct limits for each tier', () => {
+      expect(VEHICLE_LIMITS.free).toBe(2);
+      expect(VEHICLE_LIMITS.pro).toBe(5);
+      expect(VEHICLE_LIMITS.enterprise).toBeNull();
+    });
+  });
+
+  describe('getVehicleLimit', () => {
+    it('returns 2 for free tier', () => {
+      expect(getVehicleLimit('free')).toBe(2);
+    });
+
+    it('returns 5 for pro tier', () => {
+      expect(getVehicleLimit('pro')).toBe(5);
+    });
+
+    it('returns null for enterprise tier (unlimited)', () => {
+      expect(getVehicleLimit('enterprise')).toBeNull();
+    });
+  });
+
+  describe('canAddVehicle', () => {
+    describe('free tier (limit 2)', () => {
+      it('returns true when below limit', () => {
+        expect(canAddVehicle('free', 0)).toBe(true);
+        expect(canAddVehicle('free', 1)).toBe(true);
+      });
+
+      it('returns false when at limit', () => {
+        expect(canAddVehicle('free', 2)).toBe(false);
+      });
+
+      it('returns false when over limit', () => {
+        expect(canAddVehicle('free', 3)).toBe(false);
+      });
+    });
+
+    describe('pro tier (limit 5)', () => {
+      it('returns true when below limit', () => {
+        expect(canAddVehicle('pro', 0)).toBe(true);
+        expect(canAddVehicle('pro', 4)).toBe(true);
+      });
+
+      it('returns false when at limit', () => {
+        expect(canAddVehicle('pro', 5)).toBe(false);
+      });
+
+      it('returns false when over limit', () => {
+        expect(canAddVehicle('pro', 6)).toBe(false);
+      });
+    });
+
+    describe('enterprise tier (unlimited)', () => {
+      it('always returns true regardless of count', () => {
+        expect(canAddVehicle('enterprise', 0)).toBe(true);
+        expect(canAddVehicle('enterprise', 100)).toBe(true);
+        expect(canAddVehicle('enterprise', 999999)).toBe(true);
+      });
+    });
+  });
+
+  describe('getVehicleLimitConfig', () => {
+    it('returns correct config for free tier', () => {
+      const config = getVehicleLimitConfig('free');
+      expect(config.limit).toBe(2);
+      expect(config.tier).toBe('free');
+      expect(config.upgradePrompt).toContain('Free tier is limited to 2 vehicles');
+      expect(config.upgradePrompt).toContain('Pro');
+      expect(config.upgradePrompt).toContain('Enterprise');
+    });
+
+    it('returns correct config for pro tier', () => {
+      const config = getVehicleLimitConfig('pro');
+      expect(config.limit).toBe(5);
+      expect(config.tier).toBe('pro');
+      expect(config.upgradePrompt).toContain('Pro tier is limited to 5 vehicles');
+      expect(config.upgradePrompt).toContain('Enterprise');
+    });
+
+    it('returns correct config for enterprise tier', () => {
+      const config = getVehicleLimitConfig('enterprise');
+      expect(config.limit).toBeNull();
+      expect(config.tier).toBe('enterprise');
+      expect(config.upgradePrompt).toBeTruthy();
+    });
+
+    it('provides default upgradePrompt fallback', () => {
+      const config = getVehicleLimitConfig('enterprise');
+      expect(config.upgradePrompt).toBe('Upgrade to access additional vehicles.');
+    });
+  });
 });