refactor(playlists,feed): share feed-row projection & playlist helpers

- feed_columns() is the single feed-row projection, consumed by both /feed and the
  playlist-detail query (was copy-pasted, drift risk on every new column).
- _replace_items_from_youtube() dedups the YouTube playlist-mirror block shared by
  sync_user_playlists and repull_playlist.
- _next_playlist_position/_next_item_position/_add_item replace the append-position
  idiom (x4) and the exists-then-insert blocks (x2).

backend/app/routes/playlists.py

@@ -9,7 +9,7 @@ from app import quota
 from app.auth import current_user, has_read_scope, has_write_scope
 from app.db import get_db
 from app.models import Channel, Playlist, PlaylistItem, User, Video, VideoState
-from app.routes.feed import _saved_expr, _serialize
+from app.routes.feed import _serialize, feed_columns
 from app.sync.playlists import (
     plan_push,
     push_playlist,
@@ -29,6 +29,51 @@ def _own_playlist(db: Session, user: User, playlist_id: int) -> Playlist:
     return pl
 
 
+def _next_playlist_position(db: Session, user_id: int) -> int:
+    """Append position for a new playlist in this user's rail."""
+    return (
+        db.scalar(
+            select(func.coalesce(func.max(Playlist.position), -1)).where(
+                Playlist.user_id == user_id
+            )
+        )
+        + 1
+    )
+
+
+def _next_item_position(db: Session, playlist_id: int) -> int:
+    """Append position for a new item at the end of a playlist."""
+    return (
+        db.scalar(
+            select(func.coalesce(func.max(PlaylistItem.position), -1)).where(
+                PlaylistItem.playlist_id == playlist_id
+            )
+        )
+        + 1
+    )
+
+
+def _add_item(db: Session, pl: Playlist, video_id: str, *, mark_dirty: bool) -> bool:
+    """Append a video to a playlist if not already present (idempotent). Returns whether it
+    was added. `mark_dirty` recomputes the YouTube-sync dirty flag (skip for Watch later)."""
+    exists = db.scalar(
+        select(PlaylistItem).where(
+            PlaylistItem.playlist_id == pl.id, PlaylistItem.video_id == video_id
+        )
+    )
+    if exists is not None:
+        return False
+    db.add(
+        PlaylistItem(
+            playlist_id=pl.id, video_id=video_id, position=_next_item_position(db, pl.id)
+        )
+    )
+    if mark_dirty:
+        recompute_dirty(db, pl)
+    db.commit()
+    return True
+
+
 def _summary(
     db: Session,
     pl: Playlist,
@@ -142,15 +187,7 @@ def create_playlist(
     name = (payload.get("name") or "").strip()
     if not name:
         raise HTTPException(status_code=400, detail="name is required")
-    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=name, position=next_pos)
+    pl = Playlist(user_id=user.id, name=name, position=_next_playlist_position(db, user.id))
     db.add(pl)
     db.commit()
     return _summary(db, pl, 0)
@@ -164,16 +201,11 @@ def _watch_later(db: Session, user: User) -> Playlist:
         )
     )
     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
+            user_id=user.id,
+            name="Watch later",
+            kind="watch_later",
+            position=_next_playlist_position(db, user.id),
         )
         db.add(pl)
         db.commit()
@@ -191,22 +223,7 @@ def add_watch_later(
     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()
+    _add_item(db, pl, video_id, mark_dirty=False)
     return {"saved": True, "playlist_id": pl.id}
 
 
@@ -344,23 +361,7 @@ def get_playlist(
     pl = _own_playlist(db, user, playlist_id)
     state = aliased(VideoState)
     rows = db.execute(
-        select(
-            Video.id,
-            Video.title,
-            Video.channel_id,
-            Channel.title.label("channel_title"),
-            Channel.thumbnail_url.label("channel_thumbnail"),
-            Channel.handle.label("channel_handle"),
-            Video.published_at,
-            Video.thumbnail_url,
-            Video.duration_seconds,
-            Video.view_count,
-            Video.is_short,
-            Video.live_status,
-            func.coalesce(state.status, "new").label("status"),
-            func.coalesce(state.position_seconds, 0).label("position_seconds"),
-            _saved_expr(user),
-        )
+        select(*feed_columns(user, state))
         .join(PlaylistItem, PlaylistItem.video_id == Video.id)
         .join(Channel, Channel.id == Video.channel_id)
         .outerjoin(state, and_(state.video_id == Video.id, state.user_id == user.id))
@@ -439,25 +440,7 @@ def add_item(
     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")
-    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)
-        )
-        recompute_dirty(db, pl)
-        db.commit()
+    _add_item(db, pl, video_id, mark_dirty=True)
     return {"playlist_id": pl.id, "video_id": video_id}