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.

rec/throttle_detector.py

@@ -45,14 +45,17 @@ class ThrottleMetrics:
     def record_rate_limit(self) -> None:
         """Record a rate limit error (HTTP 429)."""
         self.rate_limit_count += 1
+        _increment_throttle_metric("rate_limit")
 
     def record_service_unavailable(self) -> None:
         """Record a service unavailable error (HTTP 503)."""
         self.service_unavailable_count += 1
+        _increment_throttle_metric("service_unavailable")
 
     def record_ip_blocked(self) -> None:
         """Record an IP blocked error (HTTP 403)."""
         self.ip_blocked_count += 1
+        _increment_throttle_metric("ip_blocked")
 
     def record_slow_response(self, response_time: float) -> None:
         """Record a slow response.
@@ -63,14 +66,17 @@ class ThrottleMetrics:
         self.slow_response_count += 1
         self.total_response_time += response_time
         self.total_requests += 1
+        _increment_throttle_metric("slow_response")
 
     def record_empty_response(self) -> None:
         """Record an unexpected empty response."""
         self.empty_response_count += 1
+        _increment_throttle_metric("empty_response")
 
     def record_invalid_response(self) -> None:
         """Record an invalid or error response."""
         self.invalid_response_count += 1
+        _increment_throttle_metric("invalid_response")
 
     def record_request(self, response_time: float) -> None:
         """Record a successful request.
@@ -150,6 +156,15 @@ def reset_throttle_metrics() -> None:
     _global_metrics = ThrottleMetrics()
 
 
+def _increment_throttle_metric(event_type: str) -> None:
+    """Safely increment the OTel throttle counter if metrics are initialised."""
+    try:
+        from api.metrics import throttle_events_total
+        throttle_events_total.add(1, {"type": event_type})
+    except Exception:
+        pass  # Metrics not yet initialised (e.g. during tests)
+
+
 def validate_response(
     response: aiohttp.ClientResponse,
     response_time: float,