feat(ocr): add metrics.py factory with test-scoped CollectorRegistry support

Encapsulates every custom OCR metric in an OcrMetrics frozen dataclass and
exposes a `build_metrics(registry)` factory. Production main.py binds against
the default REGISTRY; tests construct a fresh CollectorRegistry per case and
monkeypatch main.metrics, so counter values stay isolated between tests
(decision #3 on issue #652, Option A).

Refs #652

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

ocr-service/main.py

@@ -20,9 +20,11 @@ import pypdfium2 as pdfium
 from fastapi import FastAPI, Form, Header, HTTPException, UploadFile
 from fastapi.responses import StreamingResponse
 from PIL import Image
+from prometheus_client import REGISTRY
 from prometheus_fastapi_instrumentator import Instrumentator
 
 from confidence import apply_confidence_markers, get_threshold
+from metrics import OcrMetrics, build_metrics
 from spell_check import correct_text, load_spell_checker
 from engines import kraken as kraken_engine
 from engines import surya as surya_engine
@@ -38,6 +40,8 @@ logger = logging.getLogger(__name__)
 
 _models_ready = False
 
+metrics: OcrMetrics = build_metrics(REGISTRY)
+
 ALLOWED_PDF_HOSTS = set(
     h.strip() for h in os.getenv("ALLOWED_PDF_HOSTS", "minio,localhost,127.0.0.1").split(",")
 )