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>

docs/TESTING.md

@@ -74,7 +74,7 @@ docker compose exec mvp-frontend npm test -- --coverage
 
 Example: `vehicles.service.test.ts`
 - Tests VIN validation logic
-- Tests vehicle creation with mocked vPIC responses
+- Tests vehicle creation with mocked OCR service responses
 - Tests caching behavior with mocked Redis
 - Tests error handling paths
 
@@ -194,7 +194,7 @@ All 15 features have test suites with unit and/or integration tests:
 - `vehicles` - Unit + integration tests
 
 ### Mock Strategy
-- **External APIs**: Completely mocked (vPIC, Google Maps)
+- **External APIs**: Completely mocked (OCR service, Google Maps)
 - **Database**: Real database with transactions
 - **Redis**: Mocked for unit tests, real for integration
 - **Auth**: Mocked JWT tokens for protected endpoints
@@ -319,9 +319,9 @@ describe('Error Handling', () => {
     ).rejects.toThrow('Invalid VIN format');
   });
   
-  it('should handle vPIC API failure', async () => {
-    mockVpicClient.decode.mockRejectedValue(new Error('API down'));
-    
+  it('should handle OCR service failure', async () => {
+    mockOcrClient.decodeVin.mockRejectedValue(new Error('API down'));
+
     const result = await vehicleService.create(validVehicle, 'user123');
     expect(result.make).toBeNull(); // Graceful degradation
   });