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_videos.py

@@ -57,6 +57,21 @@ def test_get_video_by_youtube_id(client, db_session):
     assert body["channel"]["id"] == channel.id
     assert body["categories"] == [{"id": category.id, "name": "Linux"}]
     assert body["local"]["available"] is False
+    # 125s <= default 180s threshold.
+    assert body["is_short"] is True
+
+
+def test_get_video_is_short_false_for_long_and_unknown_duration(client, db_session, monkeypatch):
+    _seed_single_video(db_session, youtube_video_id="vidLong", duration_seconds=3600)
+    _seed_single_video(db_session, youtube_video_id="vidUnknown")
+
+    assert client.get("/api/videos/vidLong").json()["is_short"] is False
+    assert client.get("/api/videos/vidUnknown").json()["is_short"] is False
+
+    # The threshold is read from settings at serialization time.
+    monkeypatch.setattr("app.config.settings.shorts_max_duration_seconds", 4000)
+    assert client.get("/api/videos/vidLong").json()["is_short"] is True
+    assert client.get("/api/videos/vidUnknown").json()["is_short"] is False
 
 
 def test_get_video_not_found(client):
@@ -64,16 +79,19 @@ def test_get_video_not_found(client):
     assert resp.status_code == 404
 
 
-def _seed_single_video(db_session, youtube_video_id="vid1"):
-    channel = Channel(youtube_channel_id="chanA", title="Channel A", subscribed=True)
-    db_session.add(channel)
-    db_session.commit()
+def _seed_single_video(db_session, youtube_video_id="vid1", duration_seconds=None):
+    channel = db_session.query(Channel).filter(Channel.youtube_channel_id == "chanA").first()
+    if channel is None:
+        channel = Channel(youtube_channel_id="chanA", title="Channel A", subscribed=True)
+        db_session.add(channel)
+        db_session.commit()
 
     video = Video(
         youtube_video_id=youtube_video_id,
         channel_id=channel.id,
         title="Video One",
         published_at=datetime(2026, 9, 10, tzinfo=timezone.utc),
+        duration_seconds=duration_seconds,
         youtube_url=f"https://www.youtube.com/watch?v={youtube_video_id}",
     )
     db_session.add(video)