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/app/config.py

@@ -2,6 +2,10 @@ from pydantic_settings import BaseSettings
 
 
 class Settings(BaseSettings):
+    # Path Prefix
+    api_prefix: str = "/api"
+    uploads_prefix: str = "/uploads"
+
     # MongoDB
     mongodb_url: str = "mongodb://localhost:27017"
     mongodb_db_name: str = "pregnancy_report"