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.

backend/app/config.py

@@ -45,6 +45,9 @@ class Settings(BaseSettings):
     # we have already synced).
     videos_known_stop_threshold: int = 50
     new_videos_window_days: int = 2
+    # Videos with a known duration at or below this threshold (seconds) are
+    # treated as Shorts; unknown duration (NULL) counts as a regular video.
+    shorts_max_duration_seconds: int = 180
 
     log_level: str = "INFO"