fix: add proper healthchecks for Celery containers

Celery containers inherited the HTTP healthcheck from the Dockerfile
but don't run a web server. Added celery inspect ping for the worker
and schedule file check for beat.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -121,6 +121,12 @@ services:
         condition: service_healthy
       redis:
         condition: service_healthy
+    healthcheck:
+      test: ["CMD-SHELL", "celery -A config inspect ping --timeout 10 | grep -q 'pong'"]
+      interval: 30s
+      timeout: 15s
+      start_period: 40s
+      retries: 3
     cap_drop:
       - ALL
     security_opt:
@@ -146,6 +152,12 @@ services:
       - SECRET_KEY=${SECRET_KEY}
     depends_on:
       - celery-worker
+    healthcheck:
+      test: ["CMD-SHELL", "test -f /tmp/celerybeat-schedule"]
+      interval: 30s
+      timeout: 5s
+      start_period: 40s
+      retries: 3
     cap_drop:
       - ALL
     security_opt: