refactor: 移除通用频道管理(LangBot)模块，IM Bot 改为直接对接；迁移 _call_real_llm 至 im_llm

backend/python/app/api/admin/im_bot_admin.py

@@ -608,7 +608,7 @@ def _qihua_reply(text: str) -> str:
     自行创建 DB 会话，避免上层线程无可用 session 时崩溃。"""
     db = None
     try:
-        from app.api.channels import _call_real_llm
+        from app.api.im_llm import _call_real_llm
         db = SessionLocal()
         return _call_real_llm(db, None, text, "个人微信")
     except Exception as e: