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/api/feed.py

@@ -1,5 +1,6 @@
 import base64
 from datetime import datetime, timedelta, timezone
+from typing import Literal
 
 from fastapi import APIRouter, Depends, HTTPException, Query
 from sqlalchemy import func, or_, select
@@ -82,6 +83,8 @@ def get_feed(
     downloaded: bool = False,
     search: str | None = Query(None, max_length=200),
     new_only: bool = False,
+    video_type: Literal["all", "long", "short"] = Query("all", alias="type"),
+    anchor: str | None = None,
     limit: int = Query(DEFAULT_LIMIT, ge=1, le=MAX_LIMIT),
     cursor: str | None = None,
     db: Session = Depends(get_db),
@@ -118,6 +121,33 @@ def get_feed(
             or_(Video.title.ilike(needle, escape="\\"), Channel.title.ilike(needle, escape="\\"))
         )
 
+    if video_type == "short":
+        query = query.filter(
+            Video.duration_seconds.is_not(None),
+            Video.duration_seconds <= settings.shorts_max_duration_seconds,
+        )
+    elif video_type == "long":
+        query = query.filter(
+            or_(
+                Video.duration_seconds.is_(None),
+                Video.duration_seconds > settings.shorts_max_duration_seconds,
+            )
+        )
+
+    # The anchor is a one-off starting point for the Shorts feed: only the
+    # first page (no cursor) is shifted to start at that video and go older.
+    # Unknown anchors degrade silently to a regular first page.
+    if anchor and not cursor:
+        anchor_video = db.query(Video).filter(Video.youtube_video_id == anchor).first()
+        if anchor_video is not None:
+            query = query.filter(
+                (Video.published_at < anchor_video.published_at)
+                | (
+                    (Video.published_at == anchor_video.published_at)
+                    & (Video.id <= anchor_video.id)
+                )
+            )
+
     if cursor:
         cursor_published_at, cursor_id = _decode_cursor(cursor)
         query = query.filter(