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/core/config/feature-tiers.ts

@@ -26,6 +26,11 @@ export const FEATURE_TIERS: Record<string, FeatureConfig> = {
     name: 'Scan for Maintenance Schedule',
     upgradePrompt: 'Upgrade to Pro to automatically extract maintenance schedules from your vehicle manuals.',
   },
+  'vehicle.vinDecode': {
+    minTier: 'pro',
+    name: 'VIN Decode',
+    upgradePrompt: 'Upgrade to Pro to automatically decode VIN and populate vehicle details from the NHTSA database.',
+  },
 } as const;
 
 /**