feat: Add VIN decoding with NHTSA vPIC API (refs #9)

- Add NHTSA client for VIN decoding with caching and validation
- Add POST /api/vehicles/decode-vin endpoint with tier gating
- Add dropdown matching service with confidence levels
- Add decode button to VehicleForm with tier check
- Responsive layout: stacks on mobile, inline on desktop
- Only populate empty fields (preserve user input)

Backend:
- NHTSAClient with 5s timeout, VIN validation, vin_cache table
- Tier gating with 'vehicle.vinDecode' feature key (Pro+)
- Tiered matching: high (exact), medium (normalized), none

Frontend:
- Decode button with loading state and error handling
- UpgradeRequiredDialog for free tier users
- Mobile-first responsive layout

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

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

backend/src/features/vehicles/external/nhtsa/index.ts

@@ -0,0 +1,16 @@
+/**
+ * @ai-summary NHTSA vPIC integration exports
+ * @ai-context Public API for VIN decoding functionality
+ */
+
+export { NHTSAClient } from './nhtsa.client';
+export type {
+  NHTSADecodeResponse,
+  NHTSAResult,
+  DecodedVehicleData,
+  MatchedField,
+  MatchConfidence,
+  VinCacheEntry,
+  DecodeVinRequest,
+  VinDecodeError,
+} from './nhtsa.types';