chore: remove NHTSA code and update documentation (refs #227)

Delete vehicles/external/nhtsa/ directory (3 files), remove VPICVariable
and VPICResponse from platform models. Update all documentation to
reflect Gemini VIN decode via OCR service architecture.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/features/vehicles/README.md

@@ -13,7 +13,7 @@ Primary entity for vehicle management consuming MVP Platform Vehicles Service. H
 - `DELETE /api/vehicles/:id` - Soft delete vehicle
 
 ### VIN Decoding (Pro/Enterprise Only)
-- `POST /api/vehicles/decode-vin` - Decode VIN using NHTSA vPIC API
+- `POST /api/vehicles/decode-vin` - Decode VIN using Gemini via OCR service
 
 ### Hierarchical Vehicle Dropdowns
 **Status**: Vehicles service now proxies the platform vehicle catalog to provide fully dynamic dropdowns. Each selection step filters the next list, ensuring only valid combinations are shown.
@@ -104,11 +104,7 @@ vehicles/
 ├── data/                  # Database layer
 │   └── vehicles.repository.ts
 ├── external/              # External service integrations
-│   ├── CLAUDE.md          # Integration pattern docs
-│   └── nhtsa/             # NHTSA vPIC API client
-│       ├── nhtsa.client.ts
-│       ├── nhtsa.types.ts
-│       └── index.ts
+│   └── CLAUDE.md          # Integration pattern docs
 ├── migrations/            # Feature schema
 │   └── 001_create_vehicles_tables.sql
 ├── tests/                 # All tests
@@ -121,14 +117,14 @@ vehicles/
 
 ## Key Features
 
-### 🔍 VIN Decoding (NHTSA vPIC API)
+### VIN Decoding (Gemini via OCR Service)
 - **Tier Gating**: Pro and Enterprise users only (`vehicle.vinDecode` feature key)
-- **NHTSA API**: Calls official NHTSA vPIC API for authoritative vehicle data
+- **Gemini**: Calls OCR service Gemini VIN decode for authoritative vehicle data
 - **Caching**: Results cached in `vin_cache` table (1-year TTL, VIN data is static)
 - **Validation**: 17-character VIN format, excludes I/O/Q characters
 - **Matching**: Case-insensitive exact match against dropdown options
 - **Confidence Levels**: High (exact match), Medium (normalized match), None (hint only)
-- **Timeout**: 5-second timeout for NHTSA API calls
+- **Timeout**: 5-second timeout for OCR service calls
 
 #### Decode VIN Request
 ```json
@@ -140,15 +136,15 @@ Authorization: Bearer <jwt>
 
 Response (200):
 {
-  "year": { "value": 2021, "nhtsaValue": "2021", "confidence": "high" },
-  "make": { "value": "Honda", "nhtsaValue": "HONDA", "confidence": "high" },
-  "model": { "value": "Civic", "nhtsaValue": "Civic", "confidence": "high" },
-  "trimLevel": { "value": "EX", "nhtsaValue": "EX", "confidence": "high" },
-  "engine": { "value": null, "nhtsaValue": "2.0L L4 DOHC 16V", "confidence": "none" },
-  "transmission": { "value": null, "nhtsaValue": "CVT", "confidence": "none" },
-  "bodyType": { "value": null, "nhtsaValue": "Sedan", "confidence": "none" },
-  "driveType": { "value": null, "nhtsaValue": "FWD", "confidence": "none" },
-  "fuelType": { "value": null, "nhtsaValue": "Gasoline", "confidence": "none" }
+  "year": { "value": 2021, "decodedValue": "2021", "confidence": "high" },
+  "make": { "value": "Honda", "decodedValue": "HONDA", "confidence": "high" },
+  "model": { "value": "Civic", "decodedValue": "Civic", "confidence": "high" },
+  "trimLevel": { "value": "EX", "decodedValue": "EX", "confidence": "high" },
+  "engine": { "value": null, "decodedValue": "2.0L L4 DOHC 16V", "confidence": "none" },
+  "transmission": { "value": null, "decodedValue": "CVT", "confidence": "none" },
+  "bodyType": { "value": null, "decodedValue": "Sedan", "confidence": "none" },
+  "driveType": { "value": null, "decodedValue": "FWD", "confidence": "none" },
+  "fuelType": { "value": null, "decodedValue": "Gasoline", "confidence": "none" }
 }
 
 Error (400 - Invalid VIN):
@@ -157,7 +153,7 @@ Error (400 - Invalid VIN):
 Error (403 - Tier Required):
 { "error": "TIER_REQUIRED", "requiredTier": "pro", "currentTier": "free", ... }
 
-Error (502 - NHTSA Failure):
+Error (502 - OCR Service Failure):
 { "error": "VIN_DECODE_FAILED", "message": "Unable to decode VIN from external service" }
 ```
 
@@ -230,7 +226,7 @@ Error (502 - NHTSA Failure):
 ## Testing
 
 ### Unit Tests
-- `vehicles.service.test.ts` - Business logic with mocked dependencies (VIN decode, caching, CRUD operations)
+- `vehicles.service.test.ts` - Business logic with mocked dependencies (VIN decode via OCR service mock, caching, CRUD operations)
 
 ### Integration Tests
 - `vehicles.integration.test.ts` - Complete API workflow with test database (create, read, update, delete vehicles)