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/routers/decode.py

@@ -0,0 +1,67 @@
+"""VIN decode router - Gemini-powered VIN string decoding."""
+import logging
+import re
+import time
+
+from fastapi import APIRouter, HTTPException
+
+from app.engines.gemini_engine import (
+    GeminiEngine,
+    GeminiProcessingError,
+    GeminiUnavailableError,
+)
+from app.models import VinDecodeRequest, VinDecodeResponse
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/decode", tags=["decode"])
+
+_VIN_REGEX = re.compile(r"^[A-HJ-NPR-Z0-9]{17}$")
+
+# Shared engine instance (lazy init on first request)
+_gemini_engine = GeminiEngine()
+
+
+@router.post("/vin", response_model=VinDecodeResponse)
+async def decode_vin(request: VinDecodeRequest) -> VinDecodeResponse:
+    """Decode a VIN string into structured vehicle data using Gemini.
+
+    Accepts a 17-character VIN and returns year, make, model, trim, etc.
+    """
+    vin = request.vin.upper().strip()
+
+    if not _VIN_REGEX.match(vin):
+        raise HTTPException(
+            status_code=400,
+            detail=f"Invalid VIN format: must be 17 alphanumeric characters (excluding I, O, Q). Got: {vin}",
+        )
+
+    start_ms = time.monotonic_ns() // 1_000_000
+
+    try:
+        result = _gemini_engine.decode_vin(vin)
+    except GeminiUnavailableError as exc:
+        logger.error("Gemini unavailable for VIN decode: %s", exc)
+        raise HTTPException(status_code=503, detail=str(exc)) from exc
+    except GeminiProcessingError as exc:
+        logger.error("Gemini processing error for VIN %s: %s", vin, exc)
+        raise HTTPException(status_code=422, detail=str(exc)) from exc
+
+    elapsed_ms = (time.monotonic_ns() // 1_000_000) - start_ms
+
+    return VinDecodeResponse(
+        success=True,
+        vin=vin,
+        year=result.year,
+        make=result.make,
+        model=result.model,
+        trimLevel=result.trim_level,
+        bodyType=result.body_type,
+        driveType=result.drive_type,
+        fuelType=result.fuel_type,
+        engine=result.engine,
+        transmission=result.transmission,
+        confidence=result.confidence,
+        processingTimeMs=elapsed_ms,
+        error=None,
+    )