feat: 初始化孕育无忧全栈项目基础代码结构

完成项目初始搭建，包含后端FastAPI+MongoDB服务、前端React+Vite项目的完整基础结构，添加各类配置文件与依赖清单，涵盖核心业务初始框架。

backend/app/services/wechat.py

@@ -0,0 +1,26 @@
+import httpx
+
+from app.config import settings
+
+
+async def code_to_session(code: str) -> dict:
+    """用小程序 code 换取 openid 和 session_key"""
+    url = "https://api.weixin.qq.com/sns/jscode2session"
+    params = {
+        "appid": settings.wechat_app_id,
+        "secret": settings.wechat_app_secret,
+        "js_code": code,
+        "grant_type": "authorization_code",
+    }
+    async with httpx.AsyncClient() as client:
+        resp = await client.get(url, params=params)
+        data = resp.json()
+
+    if "errcode" in data and data["errcode"] != 0:
+        raise Exception(f"微信登录失败: {data.get('errmsg', '未知错误')}")
+
+    return {
+        "openid": data["openid"],
+        "session_key": data["session_key"],
+        "unionid": data.get("unionid"),
+    }