Fix celery-worker healthcheck command

Replace celery inspect ping with pgrep check for reliability.
The inspect command was failing in healthcheck context while
the worker process itself was running fine.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -127,7 +127,7 @@ services:
         condition: service_healthy
     command: celery-worker
     healthcheck:
-      test: ["CMD-SHELL", "celery -A config inspect ping -d celery@$$HOSTNAME"]
+      test: ["CMD-SHELL", "pgrep -f 'celery.*worker' || exit 1"]
       interval: 30s
       timeout: 10s
       retries: 3