feat: add IM channels for Feishu, Slack, and Telegram (#1010)

* feat: add IM channels system for Feishu, Slack, and Telegram integration

Bridge external messaging platforms to DeerFlow via LangGraph Server with
async message bus, thread management, and per-channel configuration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address review comments on IM channels system

Fix topic_id handling in store remove/list_entries and manager commands,
correct Telegram reply threading, remove unused imports/variables, update
docstrings and docs to match implementation, and prevent config mutation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* update skill creator

* fix im reply text

* fix comments

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/gateway/routers/channels.py

@@ -0,0 +1,52 @@
+"""Gateway router for IM channel management."""
+
+from __future__ import annotations
+
+import logging
+
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/api/channels", tags=["channels"])
+
+
+class ChannelStatusResponse(BaseModel):
+    service_running: bool
+    channels: dict[str, dict]
+
+
+class ChannelRestartResponse(BaseModel):
+    success: bool
+    message: str
+
+
+@router.get("/", response_model=ChannelStatusResponse)
+async def get_channels_status() -> ChannelStatusResponse:
+    """Get the status of all IM channels."""
+    from src.channels.service import get_channel_service
+
+    service = get_channel_service()
+    if service is None:
+        return ChannelStatusResponse(service_running=False, channels={})
+    status = service.get_status()
+    return ChannelStatusResponse(**status)
+
+
+@router.post("/{name}/restart", response_model=ChannelRestartResponse)
+async def restart_channel(name: str) -> ChannelRestartResponse:
+    """Restart a specific IM channel."""
+    from src.channels.service import get_channel_service
+
+    service = get_channel_service()
+    if service is None:
+        raise HTTPException(status_code=503, detail="Channel service is not running")
+
+    success = await service.restart_channel(name)
+    if success:
+        logger.info("Channel %s restarted successfully", name)
+        return ChannelRestartResponse(success=True, message=f"Channel {name} restarted successfully")
+    else:
+        logger.warning("Failed to restart channel %s", name)
+        return ChannelRestartResponse(success=False, message=f"Failed to restart channel {name}")