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.

.env.example

@@ -29,3 +29,8 @@ QINIU_CDN_DOMAIN=
 QINIU_KEY_PREFIX=uploads/
 # token 有效期（秒）
 QINIU_TOKEN_EXPIRE_SECONDS=300
+
+# 微信公众号（网页授权 OAuth2）
+WECHAT_OA_APP_ID=replace-with-oa-appid
+WECHAT_OA_APP_SECRET=replace-with-oa-appsecret
+WECHAT_OA_TIMEOUT_SECONDS=5