fix(ocr): unblock event loop during OCR and show errors in UI

OCR engines are CPU-bound and were blocking Uvicorn's single async
event loop, making /health unresponsive during processing. This caused
new OCR requests to fail silently (health check failure → no DB record
→ UI shows NONE). Wrap engine calls in asyncio.to_thread() to keep the
event loop free. Also surface OCR trigger errors in the frontend
instead of silently resetting the spinner.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ocr-service/main.py

@@ -1,5 +1,6 @@
 """OCR microservice — FastAPI app with Surya and Kraken engine support."""
 
+import asyncio
 import io
 import logging
 from contextlib import asynccontextmanager
@@ -52,6 +53,7 @@ async def run_ocr(request: OcrRequest):
 
     Downloads the PDF from the provided URL, converts pages to images,
     and runs the appropriate OCR engine based on scriptType.
+    OCR engines run in a thread pool so the event loop stays free for /health.
     """
     if not _models_ready:
         raise HTTPException(status_code=503, detail="Models not loaded yet")
@@ -66,10 +68,10 @@ async def run_ocr(request: OcrRequest):
                 status_code=400,
                 detail="Kraken model not available — cannot process Kurrent script",
             )
-        blocks = kraken_engine.extract_blocks(images, request.language)
+        blocks = await asyncio.to_thread(kraken_engine.extract_blocks, images, request.language)
     else:
         # TYPEWRITER, HANDWRITING_LATIN, UNKNOWN — all use Surya
-        blocks = surya_engine.extract_blocks(images, request.language)
+        blocks = await asyncio.to_thread(surya_engine.extract_blocks, images, request.language)
 
     threshold = get_threshold(script_type)
     for block in blocks: