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.

tasks/listing_tasks.py

@@ -23,15 +23,8 @@ from services.task_progress_publisher import publish_task_progress
 
 logger = logging.getLogger("uvicorn.error")
 
-# Also configure a celery-specific logger that always outputs to stdout
+# Central logging is now configured in celery_app.py via logging_config
 celery_logger = logging.getLogger("celery.task")
-if not celery_logger.handlers:
-    handler = logging.StreamHandler()
-    handler.setFormatter(logging.Formatter(
-        "%(asctime)s [%(levelname)s] %(name)s: %(message)s"
-    ))
-    celery_logger.addHandler(handler)
-    celery_logger.setLevel(logging.INFO)
 
 SCRAPE_LOCK_NAME = "scrape_listings"
 LOG_BUFFER_MAX_LINES = 200
@@ -574,6 +567,22 @@ async def _dump_listings_full_inner(
     )
     celery_logger.info("=" * 60)
 
+    # Record scrape metrics
+    from api.metrics import (
+        scrape_listings_found,
+        scrape_listings_processed,
+        scrape_listings_failed,
+        scrape_duration_seconds,
+        scrape_pages_fetched,
+        scrape_subqueries_total as scrape_subqueries_metric,
+    )
+    scrape_listings_found.add(state.ids_collected)
+    scrape_listings_processed.add(state.processed_count)
+    scrape_listings_failed.add(state.failed_count)
+    scrape_duration_seconds.record(elapsed)
+    scrape_pages_fetched.add(state.total_pages_fetched)
+    scrape_subqueries_metric.add(state.completed_subqueries)
+
     invalidate_cache()
 
     _update_task_state(task, "Completed", {