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/app.py

@@ -10,6 +10,7 @@ from src.gateway.config import get_gateway_config
 from src.gateway.routers import (
     agents,
     artifacts,
+    channels,
     mcp,
     memory,
     models,
@@ -47,7 +48,24 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     # 2. Gateway and LangGraph Server are separate processes with independent caches
     # MCP tools are lazily initialized in LangGraph Server when first needed
 
+    # Start IM channel service if any channels are configured
+    try:
+        from src.channels.service import start_channel_service
+
+        channel_service = await start_channel_service()
+        logger.info("Channel service started: %s", channel_service.get_status())
+    except Exception:
+        logger.exception("No IM channels configured or channel service failed to start")
+
     yield
+
+    # Stop channel service on shutdown
+    try:
+        from src.channels.service import stop_channel_service
+
+        await stop_channel_service()
+    except Exception:
+        logger.exception("Failed to stop channel service")
     logger.info("Shutting down API Gateway")
 
 
@@ -117,6 +135,10 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
                 "name": "suggestions",
                 "description": "Generate follow-up question suggestions for conversations",
             },
+            {
+                "name": "channels",
+                "description": "Manage IM channel integrations (Feishu, Slack, Telegram)",
+            },
             {
                 "name": "health",
                 "description": "Health check and system status endpoints",
@@ -151,6 +173,9 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
     # Suggestions API is mounted at /api/threads/{thread_id}/suggestions
     app.include_router(suggestions.router)
 
+    # Channels API is mounted at /api/channels
+    app.include_router(channels.router)
+
     @app.get("/health", tags=["health"])
     async def health_check() -> dict:
         """Health check endpoint.