fix: use node fetch for HEALTHCHECK (no wget in node:22-slim)

Dockerfile

@@ -14,7 +14,8 @@ RUN mkdir -p /app/generated-images
 
 EXPOSE 8935
 
-HEALTHCHECK --interval=10s --timeout=3s --retries=3 \
-  CMD wget -qO- http://127.0.0.1:8935/health || exit 1
+# HEALTHCHECK: use node itself (no curl/wget in node:22-slim)
+HEALTHCHECK --interval=15s --timeout=5s --retries=3 --start-period=10s \
+  CMD node -e "fetch('http://127.0.0.1:8935/health').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
 
 CMD ["node", "server.mjs"]