feat(ocr): flip ocr_models_ready to 1 once the lifespan startup finishes

Mirrors the existing _models_ready bool so Prometheus has a time-series
liveness/readiness signal for future alerting rules (e.g.
ocr_models_ready < 1 for 2m).

Refs #652 (AC7)

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

ocr-service/main.py

@@ -69,6 +69,7 @@ async def lifespan(app: FastAPI):
     kraken_engine.load_models()
     load_spell_checker()
     _models_ready = True
+    metrics.ocr_models_ready.set(1)
     logger.info("Startup complete — ready to accept requests")
 
     yield