Add structured JSON logging, OTel business metrics, and Grafana dashboard

Structured logging via JsonFormatter replaces uvicorn's default format so
Loki can parse timestamps and fields.  14 business metrics (scrape stats,
throttle events, circuit breaker state, cache hit rate, OCR success rate,
Celery task lifecycle) are defined in a shared metrics module and
instrumented across the scraper pipeline, API, and workers.  Celery
workers expose a Prometheus HTTP endpoint on configurable ports.

listing_processor.py

@@ -324,6 +324,15 @@ class DetectFloorplanStep(Step):
         listing.square_meters = max_sqm
         await self.listing_repository.upsert_listings([listing])
 
+        # Record OCR metrics
+        try:
+            from api.metrics import ocr_attempts, ocr_successes
+            ocr_attempts.add(1)
+            if max_sqm > 0:
+                ocr_successes.add(1)
+        except Exception:
+            pass  # Metrics not initialised
+
         if max_sqm > 0:
             logger.info(f"[{listing_id}] OCR detected {max_sqm} sqm")
         else: