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/tests/test_uploads_router.py

@@ -21,7 +21,6 @@ def test_upload_files_writes_thread_storage_and_skips_local_sandbox_sync(tmp_pat
         patch.object(uploads, "get_uploads_dir", return_value=thread_uploads_dir),
         patch.object(uploads, "get_sandbox_provider", return_value=provider),
     ):
-
         file = UploadFile(filename="notes.txt", file=BytesIO(b"hello uploads"))
         result = asyncio.run(uploads.upload_files("thread-local", files=[file]))
 
@@ -52,7 +51,6 @@ def test_upload_files_syncs_non_local_sandbox_and_marks_markdown_file(tmp_path):
         patch.object(uploads, "get_sandbox_provider", return_value=provider),
         patch.object(uploads, "convert_file_to_markdown", AsyncMock(side_effect=fake_convert)),
     ):
-
         file = UploadFile(filename="report.pdf", file=BytesIO(b"pdf-bytes"))
         result = asyncio.run(uploads.upload_files("thread-aio", files=[file]))