Split shorts from regular videos with a vertical shorts feed

- Detect shorts by duration (SHORTS_MAX_DURATION_SECONDS, default 180)
  and expose is_short in feed, video and channel-activity DTOs.
- Feed gains type=all|long|short and anchor; lists show a two-mode
  'Обычные | Shorts' filter (no 'Все') persisted in the URL.
- Clicking a short opens /shorts/🆔 a vertical scroll-snap feed with
  autoplay for the active slide, context-aware endpoints and infinite
  loading.
- Keep the sound choice across swipes, syncing with the player's own
  mute control and guarding against the widget's stale isMuted() reads.

tests/test_channel_activity.py

@@ -248,6 +248,7 @@ def test_activity_item_shape(client, db_session):
         "thumbnail_url",
         "published_at",
         "duration_seconds",
+        "is_short",
         "youtube_url",
         "categories",
         "local",