feat: add Docker deployment (backend:18889, frontend:18888)

- Dockerfile.backend: Python 3.11 + FastAPI + uvicorn
- Dockerfile.frontend: Node 20 build + Nginx serve
- docker-compose.yml: orchestration with data volume mount
- nginx.conf: API/WebSocket proxy to backend
- web/backend/requirements.txt: Python dependencies
- .dockerignore: exclude venv/node_modules/data from build

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,27 @@
+services:
+  backend:
+    build:
+      context: .
+      dockerfile: Dockerfile.backend
+    container_name: yixuan-backend
+    ports:
+      - "18889:18889"
+    volumes:
+      - ./data:/app/data
+      - ./config.ini:/app/config.ini:ro
+      - ./config:/app/config:ro
+      - ./templates:/app/templates:ro
+    env_file:
+      - .env
+    restart: unless-stopped
+
+  frontend:
+    build:
+      context: .
+      dockerfile: Dockerfile.frontend
+    container_name: yixuan-frontend
+    ports:
+      - "18888:18888"
+    depends_on:
+      - backend
+    restart: unless-stopped