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>

Dockerfile

@@ -35,4 +35,7 @@ USER svelte
 EXPOSE 3001
 ENV PORT=3001
 
+HEALTHCHECK --interval=5s --timeout=3s --start-period=15s --retries=3 \
+  CMD wget -qO- http://localhost:3001/ > /dev/null || exit 1
+
 CMD ["node", "build"]

docker-compose.prod.yml

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