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/docs/TODO.md

@@ -20,6 +20,13 @@
 - [ ] Add metrics and monitoring
 - [ ] Support for more document formats in upload
 - [ ] Skill marketplace / remote skill installation
+- [ ] Optimize async concurrency in agent hot path (IM channels multi-task scenario)
+  - Replace `time.sleep(5)` with `asyncio.sleep()` in `src/tools/builtins/task_tool.py` (subagent polling)
+  - Replace `subprocess.run()` with `asyncio.create_subprocess_shell()` in `src/sandbox/local/local_sandbox.py`
+  - Replace sync `requests` with `httpx.AsyncClient` in community tools (tavily, jina_ai, firecrawl, infoquest, image_search)
+  - Replace sync `model.invoke()` with async `model.ainvoke()` in title_middleware and memory updater
+  - Consider `asyncio.to_thread()` wrapper for remaining blocking file I/O
+  - For production: use `langgraph up` (multi-worker) instead of `langgraph dev` (single-worker)
 
 ## Resolved Issues