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/services/video_presentation.py

@@ -1,6 +1,7 @@
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
+from app.config import settings
 from app.models.category import Category
 from app.models.channel import Channel
 from app.models.channel_category import channel_categories
@@ -51,6 +52,8 @@ def serialize_video(
         "thumbnail_url": video.thumbnail_url,
         "published_at": video.published_at,
         "duration_seconds": video.duration_seconds,
+        "is_short": video.duration_seconds is not None
+        and video.duration_seconds <= settings.shorts_max_duration_seconds,
         "youtube_url": video.youtube_url,
         "categories": categories,
         "local": _serialize_local(download_job),