Integrate WeChat Official Account support and update configuration

- Added WeChat Official Account configuration options to .env.example and config.go for OAuth2 integration.
- Updated main.go to initialize WeChat OAuth handler and register routes for handling OAuth requests.
- Enhanced documentation to include references for WeChat Official Account functionality.
- Updated route registration to accommodate the new OAuth handler for improved API structure.

docs/common/README.md

@@ -26,3 +26,7 @@
 ## 上传（七牛直传）
 
 - `docs/common/upload_qiniu.md`
+
+## 微信公众号
+
+- `docs/common/wechat_official.md`

docs/common/wechat_official.md

@@ -0,0 +1,65 @@
+# 微信公众号：code 换取 openid/用户信息
+
+本接口用于微信公众号网页授权（OAuth2）：前端拿到微信回调的 `code` 后，调用后端换取 `openid`，并可选获取用户信息（昵称/头像/unionid 等）。
+https://open.weixin.qq.com/connect/oauth2/authorize?appid=wx65d83a301fc84068&redirect_uri=https%3A%2F%2Fwww.baidu.com&response_type=code&scope=&state=1#wechat_redirect
+https://open.weixin.qq.com/connect/oauth2/authorize?appid=wx65d83a301fc84068&redirect_uri=https%3A%2F%2Fwww.baidu.com&response_type=code&scope=SCOPE&state=STATE#wechat_redirect
+## 配置
+
+在 `.env` 中配置（示例见：`.env.example`）：
+
+- `WECHAT_OA_APP_ID`
+- `WECHAT_OA_APP_SECRET`
+- `WECHAT_OA_TIMEOUT_SECONDS`（可选）
+
+## 接口
+
+`POST /api/v1/wechat/official/oauth/code2user`
+
+说明：
+- 不需要登录（无需 Bearer Token），因为该 `code` 本身来自微信网页授权流程。
+
+请求体：
+
+```json
+{
+  "code": "微信回调参数 code",
+  "with_userinfo": true
+}
+```
+
+- `with_userinfo` 可选，默认 `true`。
+- 若网页授权 scope 仅为 `snsapi_base`，通常只能拿到 `openid`，获取 `userinfo` 可能失败（接口会返回 `userinfo_error` 提示）。
+
+curl 示例：
+
+```bash
+curl -X POST 'http://127.0.0.1:8080/api/v1/wechat/official/oauth/code2user' \
+  -H 'Content-Type: application/json' \
+  -d '{"code":"YOUR_CODE","with_userinfo":true}'
+```
+
+成功响应示例（节选）：
+
+```json
+{
+  "code": 200,
+  "message": "success",
+  "data": {
+    "openid": "oXXXX",
+    "unionid": "oXXXX",
+    "scope": "snsapi_userinfo",
+    "expires_in": 7200,
+    "userinfo": {
+      "openid": "oXXXX",
+      "nickname": "昵称",
+      "headimgurl": "https://...",
+      "sex": 1,
+      "province": "Guangdong",
+      "city": "Shenzhen",
+      "country": "CN",
+      "unionid": "oXXXX"
+    }
+  }
+}
+```
+