fix(docker): add healthcheck and stop grace period

Watchtower waits for container to be healthy before stopping the old
one. Grace period allows in-flight requests to drain on shutdown.

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

docker-compose.prod.yml

@@ -10,6 +10,7 @@ services:
       - image-cache:/data/image-cache
     networks:
       - web
+    stop_grace_period: 30s
 
 volumes:
   image-cache: