feat(channels): add video support for Telegram and WebSocket

Telegram previously sent all video files as documents via send_document,
so users saw a file icon instead of an inline player. WebSocket only
accepted image MIME types, rejecting video uploads entirely.

Telegram:
- Recognize video extensions (mp4/mov/avi/mkv/webm/3gp) in _get_media_type
- Route videos through send_video with supports_streaming=True
- Add VIDEO/VIDEO_NOTE/ANIMATION to inbound message filters
- Add video MIME mappings to _get_extension
- Fix: local file sends now use _call_with_retry (previously no retry)

WebSocket:
- Expand upload MIME whitelist with video/mp4, video/webm, video/quicktime
- Add per-type size limits (_MAX_VIDEO_BYTES=20MB, _MAX_VIDEOS_PER_MESSAGE=1)
- Expand media serving endpoint to serve video with correct Content-Type

Agent:
- Add "video" to message tool media parameter description
- Add .mp4 example to identity.md system prompt

Made-with: Cursor

tests/channels/test_telegram_channel.py

@@ -59,6 +59,9 @@ class _FakeBot:
     async def send_photo(self, **kwargs) -> None:
         self.sent_media.append({"kind": "photo", **kwargs})
 
+    async def send_video(self, **kwargs) -> None:
+        self.sent_media.append({"kind": "video", **kwargs})
+
     async def send_voice(self, **kwargs) -> None:
         self.sent_media.append({"kind": "voice", **kwargs})