feat(playlists): unify Saved into a built-in Watch later playlist

The old per-video status='saved' becomes membership in a built-in, undeletable
'watch_later' playlist. Migration 0012 moves existing saved videos into each user's
Watch later list and demotes the states to 'new'. The feed/playlist serializers now
expose a 'saved' boolean (EXISTS in watch_later); the card bookmark toggles watch_later
via new /api/playlists/watch-later add/remove endpoints (create-on-demand) instead of
setting a status. Removed the 'saved' show-filter and status. Watch later shows a
localized name and hides rename/delete in the Playlists page and add-to-playlist popover.

backend/app/routes/playlists.py

@@ -8,7 +8,7 @@ from sqlalchemy.orm import Session, aliased
 from app.auth import current_user
 from app.db import get_db
 from app.models import Channel, Playlist, PlaylistItem, User, Video, VideoState
-from app.routes.feed import _serialize
+from app.routes.feed import _saved_expr, _serialize
 
 router = APIRouter(prefix="/api/playlists", tags=["playlists"])
 
@@ -110,6 +110,83 @@ def create_playlist(
     return _summary(db, pl, 0)
 
 
+def _watch_later(db: Session, user: User) -> Playlist:
+    """The user's built-in Watch later playlist, created on first use."""
+    pl = db.scalar(
+        select(Playlist).where(
+            Playlist.user_id == user.id, Playlist.kind == "watch_later"
+        )
+    )
+    if pl is None:
+        next_pos = (
+            db.scalar(
+                select(func.coalesce(func.max(Playlist.position), -1)).where(
+                    Playlist.user_id == user.id
+                )
+            )
+            + 1
+        )
+        pl = Playlist(
+            user_id=user.id, name="Watch later", kind="watch_later", position=next_pos
+        )
+        db.add(pl)
+        db.commit()
+    return pl
+
+
+@router.post("/watch-later")
+def add_watch_later(
+    payload: dict,
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Bookmark shortcut: add a video to the built-in Watch later playlist."""
+    video_id = payload.get("video_id")
+    if not video_id or db.get(Video, video_id) is None:
+        raise HTTPException(status_code=404, detail="Unknown video")
+    pl = _watch_later(db, user)
+    exists = db.scalar(
+        select(PlaylistItem).where(
+            PlaylistItem.playlist_id == pl.id, PlaylistItem.video_id == video_id
+        )
+    )
+    if exists is None:
+        next_pos = (
+            db.scalar(
+                select(func.coalesce(func.max(PlaylistItem.position), -1)).where(
+                    PlaylistItem.playlist_id == pl.id
+                )
+            )
+            + 1
+        )
+        db.add(PlaylistItem(playlist_id=pl.id, video_id=video_id, position=next_pos))
+        db.commit()
+    return {"saved": True, "playlist_id": pl.id}
+
+
+@router.delete("/watch-later/{video_id}")
+def remove_watch_later(
+    video_id: str,
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    pl = db.scalar(
+        select(Playlist).where(
+            Playlist.user_id == user.id, Playlist.kind == "watch_later"
+        )
+    )
+    if pl is not None:
+        item = db.scalar(
+            select(PlaylistItem).where(
+                PlaylistItem.playlist_id == pl.id, PlaylistItem.video_id == video_id
+            )
+        )
+        if item is not None:
+            db.delete(item)
+            db.commit()
+    return {"saved": False}
+
+
 @router.get("/{playlist_id}")
 def get_playlist(
     playlist_id: int,
@@ -134,6 +211,7 @@ def get_playlist(
             Video.live_status,
             func.coalesce(state.status, "new").label("status"),
             func.coalesce(state.position_seconds, 0).label("position_seconds"),
+            _saved_expr(user),
         )
         .join(PlaylistItem, PlaylistItem.video_id == Video.id)
         .join(Channel, Channel.id == Video.channel_id)