feat(gateway-chat): persistent static-serve stack for the model-smoking web chat

Stands up tools/gateway-chat.html as a permanent URL on ana-docker (http://10.250.50.70:8091)
via a tiny nginx:alpine static container (no GPU, no DB). conf/index.html is a deployed
mirror of tools/gateway-chat.html (re-sync one-liner in README). Homepage tile + tnet per
convention. The enhanced tool (auto-discovers /v1/models, system prompts, streaming +
reasoning, image upload for vision) is now always-on for smoking new gateway models.

stacks/gateway-chat/compose.yaml

@@ -0,0 +1,37 @@
+# gateway-chat — persistent static-serve of the zero-dep web chat for SMOKING models on
+# the LiteLLM gateway (auto-discovers /v1/models, system prompts, streaming +
+# reasoning_content, image upload for vision models). Tiny nginx:alpine serving ONE file;
+# no GPU, no DB. The served file is conf/index.html — a deployed mirror of the repo's
+# tools/gateway-chat.html (re-sync on edit; see README).
+#
+# Tunables in .env.
+
+name: gateway-chat
+
+services:
+  gateway-chat:
+    image: ${GATEWAYCHAT_IMAGE:-nginx:alpine}
+    container_name: ${GATEWAYCHAT_CONTAINER_NAME:-gateway-chat}
+    restart: unless-stopped
+    ports:
+      - "${GATEWAYCHAT_PORT:-8091}:80"
+    volumes:
+      - /opt/docker/conf/gateway-chat/index.html:/usr/share/nginx/html/index.html:ro
+    healthcheck:
+      test: ["CMD-SHELL", "wget -qO /dev/null http://localhost/ || exit 1"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+    networks:
+      - tnet
+    labels:
+      - homepage.group=AI Systems
+      - homepage.name=Gateway Chat
+      - homepage.icon=mdi-chat-processing
+      - homepage.description=Light web chat for smoking LiteLLM gateway models (auto-discovery, system prompts, vision)
+      - homepage.href=http://10.250.50.70:${GATEWAYCHAT_PORT:-8091}
+
+networks:
+  tnet:
+    name: traefik-net
+    external: true