feat: 新增可配置的路径前缀以支持子路径部署

添加前后端环境变量配置示例，新增后端路径前缀配置项，更新后端路由与静态文件挂载逻辑，调整前端代理配置与 API 调用代码，适配子路径部署场景。

backend/.env.example

@@ -1,29 +1,37 @@
-# MongoDB
-MONGODB_URL=mongodb://localhost:27017
-MONGODB_DB_NAME=pregnancy_report
+# Path Prefix
+API_PREFIX=/app_03/api
+UPLOADS_PREFIX=/uploads
+
+# MongoDB (通过 SSH 隧道连接到服务器)
+# MONGODB_URL=mongodb+srv://shutiaozhao127_db_user:YEJzZUi0nbEuFYL3@cluster0.bkztgqx.mongodb.net/?appName=Cluster0
+# MONGODB_DB_NAME=pregnancy_report
+MONGODB_URL=mongodb://root:mgdb4link@localhost:27017/?authSource=admin
+MONGODB_DB_NAME=app_03_dev
 
 # JWT
-JWT_SECRET_KEY=your-secret-key-change-this
+JWT_SECRET_KEY=pregnancy_ai_demo_666888
 JWT_ALGORITHM=HS256
 JWT_EXPIRE_MINUTES=10080
 
 # WeChat Mini Program
-WECHAT_APP_ID=your-wechat-app-id
-WECHAT_APP_SECRET=your-wechat-app-secret
+WECHAT_APP_ID=
+WECHAT_APP_SECRET=
 
-# Claude AI
-ANTHROPIC_API_KEY=your-anthropic-api-key
-ANTHROPIC_BASE_URL=
-AI_MODEL_NAME=claude-sonnet-4-20250514
+# AI API (硅基流动 - OpenAI 兼容接口)
+AI_API_KEY=sk-pnbkufcvwbnmzcwdhwzecdgpiqqsumtsnfojtozmmrfqiuil
+AI_BASE_URL=https://api.siliconflow.cn/v1
+AI_VISION_MODEL=Qwen/Qwen2.5-VL-72B-Instruct
+AI_TEXT_MODEL=Qwen/Qwen3-235B-A22B-Instruct-2507
 
 # CORS（多个域名逗号分隔，* 表示允许所有）
 CORS_ORIGINS=*
 
+
 # File Upload
 UPLOAD_DIR=./uploads
 MAX_FILE_SIZE=10485760
 
 # Server
 HOST=0.0.0.0
-PORT=8000
+PORT=8030
 DEBUG=true