feat: 集成 DeepSeek AI 服务

- 新增 backend 服务 (Express + TypeScript)
- 前端表单提交改为调用后端 API
- DeepSeek 增强分析结果，本地规则作为 fallback
- Docker 双容器部署：web (Nginx) + backend
- Nginx 反向代理 /api 到后端服务
- 本地开发支持 Vite 代理 + backend:dev 脚本

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

backend/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "tianxuan-company-backend",
+  "private": true,
+  "version": "0.1.0",
+  "type": "module",
+  "scripts": {
+    "dev": "tsx watch src/server.ts",
+    "start": "node --import tsx src/server.ts"
+  },
+  "dependencies": {
+    "express": "^5.1.0"
+  },
+  "devDependencies": {
+    "@types/express": "^5.0.3",
+    "tsx": "^4.20.6",
+    "typescript": "^5.9.2"
+  }
+}