Fix metrics not initialized in Celery prefork worker processes

worker_ready fires in the main process after pool workers are forked,
so init_metrics() never ran in the child processes. Add a
worker_process_init handler to initialize OTel metrics in each worker.

celery_app.py

@@ -1,7 +1,7 @@
 import sys
 import time
 from celery import Celery
-from celery.signals import worker_ready, task_prerun, task_postrun
+from celery.signals import worker_ready, worker_process_init, task_prerun, task_postrun
 from dotenv import load_dotenv
 import os
 
@@ -35,9 +35,16 @@ CELERY_METRICS_PORT = int(os.getenv("CELERY_METRICS_PORT", "9090"))
 _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"))
+
+
 @worker_ready.connect
 def _start_metrics_server(**kwargs: object) -> None:
-    """Start a lightweight Prometheus HTTP server in the worker process."""
+    """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"))