feat: add VIN decode endpoint to OCR Python service (refs #224)

Add POST /decode/vin endpoint using Gemini 2.5 Flash for VIN string
decoding. Returns structured vehicle data (year, make, model, trim,
body/drive/fuel type, engine, transmission) with confidence score.

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

ocr/app/models/__init__.py

@@ -14,6 +14,8 @@ from .schemas import (
     ReceiptExtractedField,
     ReceiptExtractionResponse,
     VinAlternative,
+    VinDecodeRequest,
+    VinDecodeResponse,
     VinExtractionResponse,
 )
 
@@ -32,5 +34,7 @@ __all__ = [
     "ReceiptExtractedField",
     "ReceiptExtractionResponse",
     "VinAlternative",
+    "VinDecodeRequest",
+    "VinDecodeResponse",
     "VinExtractionResponse",
 ]