feat: rewire vehicles controller to OCR VIN decode (refs #226)

Replace NHTSAClient with OcrClient in vehicles controller. Move cache
logic into VehiclesService with format-aware reads (Gemini vs legacy
NHTSA entries). Rename nhtsaValue to sourceValue in MatchedField.
Remove vpic config from Zod schema and YAML config files.

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

backend/src/core/config/config-loader.ts

@@ -41,14 +41,6 @@ const configSchema = z.object({
     audience: z.string(),
   }),
 
-  // External APIs configuration (optional)
-  external: z.object({
-    vpic: z.object({
-      url: z.string(),
-      timeout: z.string(),
-    }).optional(),
-  }).optional(),
-
   // Service configuration
   service: z.object({
     name: z.string(),

backend/src/core/config/feature-tiers.ts

@@ -29,7 +29,7 @@ export const FEATURE_TIERS: Record<string, FeatureConfig> = {
   'vehicle.vinDecode': {
     minTier: 'pro',
     name: 'VIN Decode',
-    upgradePrompt: 'Upgrade to Pro to automatically decode VIN and populate vehicle details from the NHTSA database.',
+    upgradePrompt: 'Upgrade to Pro to automatically decode VIN and populate vehicle details from the vehicle database.',
   },
   'fuelLog.receiptScan': {
     minTier: 'pro',