feat: add WeChat channel integration (#1869)

* feat: add WeChat channel integration

* fix(backend): recover stale channel threads and align upload artifact handling

* refactor(wechat): reduce scope and restore QR bootstrap

* fix(backend): sort manager imports for Ruff lint

* fix(tests): add missing patch import in test_channels.py

* Update backend/app/channels/wechat.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update backend/app/channels/manager.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix(wechat): streamline allowed file extensions initialization and clean up test file

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/app/channels/service.py

@@ -18,6 +18,7 @@ _CHANNEL_REGISTRY: dict[str, str] = {
     "feishu": "app.channels.feishu:FeishuChannel",
     "slack": "app.channels.slack:SlackChannel",
     "telegram": "app.channels.telegram:TelegramChannel",
+    "wechat": "app.channels.wechat:WechatChannel",
     "wecom": "app.channels.wecom:WeComChannel",
 }