fix(playlists): lock the playlist row before reading and writing item positions

Two adds read the same max(position) and a reorder racing an add collided with
it. Every add/insert/reorder takes the parent row FOR UPDATE; the YouTube list
breaks position ties by id and a Plex reorder renumbers rows it was not sent.

backend/app/routes/plex/playlists.py

@@ -6,7 +6,7 @@ from fastapi import APIRouter, Depends
 from typing import Annotated
 
 from pydantic import BaseModel, Field, model_validator
-from sqlalchemy import delete, func, select, update
+from sqlalchemy import delete, func, select
 from sqlalchemy.orm import Session
 
 from app.auth import current_user
@@ -219,7 +219,8 @@ def playlist_add_item(
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist")
+    # Locked (like every add / reorder here): two adds otherwise read the same max position.
+    pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist", lock=True)
     it = by_rating_key(db, PlexItem, body.item_rating_key)
     if it is None:
         raise not_found("item")
@@ -337,7 +338,7 @@ def playlist_add_bulk(
     """Append many items at once (a whole season/show). Already-present items are skipped; the input
     order is preserved for the newly-added ones. With `positions`, each key is put back at its index
     instead of appended. Returns how many were added + the new total."""
-    pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist")
+    pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist", lock=True)
     if body.positions is not None:
         by_rk = {it.rating_key: it for it in _items_by_rks(db, body.item_rating_keys)}
         placed = [
@@ -428,7 +429,7 @@ def reorder_playlist(
     db: Session = Depends(get_db),
 ) -> dict:
     """Set the playlist order from an explicit list of item rating_keys (0-based positions)."""
-    pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist")
+    pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist", lock=True)
     order = body.item_rating_keys
     id_by_rk = {
         it.rating_key: it.id
@@ -436,14 +437,27 @@ def reorder_playlist(
             select(PlexItem).where(PlexItem.rating_key.in_(order or [""]))
         ).scalars()
     }
-    for pos, rk in enumerate(order):
-        iid = id_by_rk.get(rk)
-        if iid is not None:
-            db.execute(
-                update(PlexPlaylistItem)
-                .where(PlexPlaylistItem.playlist_id == pid, PlexPlaylistItem.item_id == iid)
-                .values(position=pos)
-            )
+    rows = {
+        r.item_id: r
+        for r in db.execute(
+            select(PlexPlaylistItem).where(PlexPlaylistItem.playlist_id == pid)
+        ).scalars()
+    }
+    pos = 0
+    seen: set[int] = set()
+    for rk in order:
+        row = rows.get(id_by_rk.get(rk, -1))
+        if row is not None and row.item_id not in seen:
+            row.position = pos
+            pos += 1
+            seen.add(row.item_id)
+    # Rows the payload omitted (added in another tab meanwhile) keep their relative order after
+    # the given ones, so no two rows share a position (the YouTube reorder does the same).
+    for row in sorted(
+        (r for iid, r in rows.items() if iid not in seen), key=lambda r: (r.position, r.id)
+    ):
+        row.position = pos
+        pos += 1
     pl.updated_at = datetime.now(timezone.utc)
     db.commit()
     return {"ok": True}