perf: 性能与资源优化 - 提取内联资源、数据库查询优化、限流与缓存增强

主要优化:
- templates/index.html: 220KB → 28KB，内联CSS/JS提取为静态文件
- menu/serializers.py: 修复 Order.ingredients_summary N+1 查询问题
- menu/services/dish_service.py: 移除多余查询，添加 only() 限制字段
- menu/views.py: 上传接口添加每分钟20次限流
- static/js/api.js: 统一API错误处理
- zhangmenu/settings.py: 添加Whitenoise压缩、Redis缓存支持、环境变量配置
- static/css/app.css, static/js/app.js: 提取的内联资源文件
- 新增 migrations, requirements.txt, .env.example

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

.env.example

@@ -0,0 +1,17 @@
+# Django Secret Key (generate a new one for production!)
+DJANGO_SECRET_KEY=your-secret-key-here
+
+# Debug mode (set to false in production!)
+DJANGO_DEBUG=false
+
+# Allowed hosts (comma-separated)
+DJANGO_ALLOWED_HOSTS=localhost,127.0.0.1,your-domain.com
+
+# Database (PostgreSQL recommended for production)
+# DATABASE_URL=postgres://user:password@localhost:5432/zhangmenu
+
+# Redis cache (recommended for production)
+# REDIS_URL=redis://localhost:6379/1
+
+# CORS allowed origins (comma-separated)
+# CORS_ALLOWED_ORIGINS=http://localhost:8080,http://your-domain.com