feat(02-05): Slack file_share extraction and channel-aware outbound routing

- Add gateway/channels/slack_media.py with is_file_share_event, media_type_from_mime,
  build_slack_storage_key, build_attachment_from_slack_file, download_and_store_slack_file
- Add _send_response() helper to orchestrator/tasks.py for channel-aware dispatch
  (Slack -> chat.update, WhatsApp -> send_whatsapp_message)
- Add send_whatsapp_message import to orchestrator/tasks.py for WhatsApp outbound
- Add boto3>=1.35.0 to gateway dependencies for MinIO S3 client
- Add 23 unit tests in test_slack_media.py (TDD)

packages/gateway/pyproject.toml

@@ -16,6 +16,7 @@ dependencies = [
     "python-telegram-bot>=21.0",
     "httpx>=0.28.0",
     "redis>=5.0.0",
+    "boto3>=1.35.0",
 ]
 
 [tool.uv.sources]