feat(ocr): increment ocr_jobs_total with engine and script_type labels

Pick engine="kraken" for HANDWRITING_KURRENT, engine="surya" otherwise,
then increment after the blocks have been extracted.

Refs #652 (AC2)

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

ocr-service/main.py

@@ -106,6 +106,7 @@ async def run_ocr(request: OcrRequest):
         del img
 
     script_type = request.scriptType.upper()
+    engine_name = "kraken" if script_type == "HANDWRITING_KURRENT" else "surya"
 
     if script_type == "HANDWRITING_KURRENT":
         if not kraken_engine.is_available():
@@ -119,6 +120,8 @@ async def run_ocr(request: OcrRequest):
         # TYPEWRITER, HANDWRITING_LATIN, UNKNOWN — all use Surya
         blocks = await asyncio.to_thread(surya_engine.extract_blocks, images, request.language)
 
+    metrics.ocr_jobs_total.labels(engine=engine_name, script_type=script_type).inc()
+
     threshold = get_threshold(script_type)
     for block in blocks:
         if block.get("words"):