Fix celery metrics not reaching Prometheus and update Grafana dashboard

Init OTel metrics at module level in celery_app.py so prefork child
processes inherit the MeterProvider and PrometheusMetricReader from
the parent.  Previously, worker_process_init created a separate
MeterProvider in each child, disconnected from the HTTP server in
the main process — so all scrape/celery/OCR metrics were silently
lost.

Update Grafana dashboard with API Performance and Frontend Performance
sections, synced from the live cluster dashboard.

celery_app.py

@@ -1,7 +1,7 @@
 import sys
 import time
 from celery import Celery
-from celery.signals import worker_ready, worker_process_init, task_prerun, task_postrun
+from celery.signals import worker_ready, task_prerun, task_postrun
 from dotenv import load_dotenv
 import os
 
@@ -34,20 +34,17 @@ CELERY_METRICS_PORT = int(os.getenv("CELERY_METRICS_PORT", "9090"))
 # Track task start times for duration measurement
 _task_start_times: dict[str, float] = {}
 
-
-@worker_process_init.connect
-def _init_worker_metrics(**kwargs: object) -> None:
-    """Initialise OTel metrics in each prefork worker process."""
-    from api.metrics import init_metrics
-    init_metrics(os.getenv("SERVICE_NAME", "celery-worker"))
+# Initialise OTel metrics at module level so prefork children inherit the
+# MeterProvider and PrometheusMetricReader.  The prometheus_client collectors
+# are registered in the default registry before fork, so child-process
+# recordings are visible to the HTTP server started in the main process.
+from api.metrics import init_metrics as _init_metrics  # noqa: E402
+_init_metrics(os.getenv("SERVICE_NAME", "celery-worker"))
 
 
 @worker_ready.connect
 def _start_metrics_server(**kwargs: object) -> None:
     """Start a lightweight Prometheus HTTP server in the main worker process."""
-    from api.metrics import init_metrics
-    init_metrics(os.getenv("SERVICE_NAME", "celery-worker"))
-
     from prometheus_client import start_http_server
     start_http_server(CELERY_METRICS_PORT)