feat(ocr): expose /metrics endpoint via prometheus-fastapi-instrumentator

Mount the instrumentator immediately after FastAPI app creation, excluding
/health and /metrics from request metrics to keep http_requests_total focused
on real application traffic.

Refs #652

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

ocr-service/main.py

@@ -20,6 +20,7 @@ import pypdfium2 as pdfium
 from fastapi import FastAPI, Form, Header, HTTPException, UploadFile
 from fastapi.responses import StreamingResponse
 from PIL import Image
+from prometheus_fastapi_instrumentator import Instrumentator
 
 from confidence import apply_confidence_markers, get_threshold
 from spell_check import correct_text, load_spell_checker
@@ -72,6 +73,8 @@ async def lifespan(app: FastAPI):
 
 app = FastAPI(title="Familienarchiv OCR Service", lifespan=lifespan)
 
+Instrumentator(excluded_handlers=["/health", "/metrics"]).instrument(app).expose(app)
+
 
 @app.get("/health")
 def health():