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/feature-tiers.ts

@@ -76,3 +76,75 @@ export function getFeatureConfig(featureKey: string): FeatureConfig | undefined
 export function getAllFeatureConfigs(): Record<string, FeatureConfig> {
   return { ...FEATURE_TIERS };
 }
+
+// Vehicle limits per tier
+// null indicates unlimited (enterprise tier)
+export const VEHICLE_LIMITS: Record<SubscriptionTier, number | null> = {
+  free: 2,
+  pro: 5,
+  enterprise: null,
+} as const;
+
+/**
+ * Vehicle limits vary by subscription tier and must be queryable
+ * at runtime for both backend enforcement and frontend UI state.
+ *
+ * @param tier - User's subscription tier
+ * @returns Maximum vehicles allowed, or null for unlimited (enterprise tier)
+ */
+export function getVehicleLimit(tier: SubscriptionTier): number | null {
+  return VEHICLE_LIMITS[tier] ?? null;
+}
+
+/**
+ * Check if a user can add another vehicle based on their tier and current count.
+ *
+ * @param tier - User's subscription tier
+ * @param currentCount - Number of vehicles user currently has
+ * @returns true if user can add another vehicle, false if at/over limit
+ */
+export function canAddVehicle(tier: SubscriptionTier, currentCount: number): boolean {
+  const limit = getVehicleLimit(tier);
+  // null limit means unlimited (enterprise)
+  if (limit === null) {
+    return true;
+  }
+  return currentCount < limit;
+}
+
+/**
+ * Vehicle limit configuration with upgrade prompt.
+ * Structure supports additional resource types in the future.
+ */
+export interface VehicleLimitConfig {
+  limit: number | null;
+  tier: SubscriptionTier;
+  upgradePrompt: string;
+}
+
+/**
+ * Get vehicle limit configuration with upgrade prompt for a tier.
+ *
+ * @param tier - User's subscription tier
+ * @returns Configuration with limit and upgrade prompt
+ */
+export function getVehicleLimitConfig(tier: SubscriptionTier): VehicleLimitConfig {
+  const limit = getVehicleLimit(tier);
+
+  const defaultPrompt = 'Upgrade to access additional vehicles.';
+
+  let upgradePrompt: string;
+  if (tier === 'free') {
+    upgradePrompt = 'Free tier is limited to 2 vehicles. Upgrade to Pro for up to 5 vehicles, or Enterprise for unlimited.';
+  } else if (tier === 'pro') {
+    upgradePrompt = 'Pro tier is limited to 5 vehicles. Upgrade to Enterprise for unlimited vehicles.';
+  } else {
+    upgradePrompt = defaultPrompt;
+  }
+
+  return {
+    limit,
+    tier,
+    upgradePrompt,
+  };
+}