fix(playlists): a push stops at a list deleted mid-way, and a rename made during it stays unsynced

A delete landing while a push read YouTube left a plan over no items, so the push emptied the
YouTube playlist a local-only delete keeps: the push now stops at a vanished list (PlaylistGone,
404), and a delete is refused while a sync of the list runs. The closing baseline fingerprints the
title YouTube got, not the one its lock re-reads, so a rename made during the writes stays dirty.
The other-tab test helper bounds its lock wait, so a lock-order regression fails instead of hanging.

backend/app/routes/playlists.py

@@ -14,6 +14,7 @@ from app.db import get_db
 from app.feedquery import feed_columns, serialize
 from app.models import Channel, Playlist, PlaylistItem, User, Video, VideoState
 from app.sync.playlists import (
+    PlaylistGone,
     lock_playlists,
     merge_playlist,
     plan_merge,
@@ -24,7 +25,7 @@ from app.sync.playlists import (
     user_sync_claim,
 )
 from app.ratelimit import RateLimiter
-from app.routes._common import get_or_404, owned_or_404, youtube_refusal
+from app.routes._common import get_or_404, not_found, owned_or_404, youtube_refusal
 from app.youtube.client import YouTubeClient, YouTubeError
 
 router = APIRouter(prefix="/api/playlists", tags=["playlists"])
@@ -367,6 +368,8 @@ def revert_youtube(
         try:
             with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
                 return {"ok": True, **repull_playlist(db, user, pl)}
+        except PlaylistGone:
+            raise not_found("playlist")
         except YouTubeError as exc:
             db.rollback()
             raise youtube_refusal(exc)
@@ -432,6 +435,8 @@ def push(
                 if plan["units_estimate"] > quota.remaining_today(db):
                     raise _refused(429, "quota_exceeded")
                 return {"ok": True, **merge_playlist(db, user, pl, live=live)}
+        except PlaylistGone:
+            raise not_found("playlist")
         except YouTubeError as exc:
             db.rollback()
             raise youtube_refusal(exc)
@@ -517,21 +522,28 @@ def delete_playlist(
     pl = owned_or_404(db, Playlist, playlist_id, user, what="playlist")
     if pl.kind == "watch_later":
         raise _refused(400, "watch_later_undeletable")
-    if on_youtube:
-        if not pl.yt_playlist_id:
-            raise _refused(400, "not_on_youtube")
-        if not has_write_scope(user):
-            raise _refused(403, "youtube_editing_off")
-        try:
-            with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_DELETE):
-                with YouTubeClient(db, user) as yt:
-                    yt.delete_playlist(pl.yt_playlist_id)
-        except YouTubeError as exc:
-            raise youtube_refusal(exc)
-    # The playlist first, as every item write: the cascade deletes the items before the list row.
-    lock_playlists(db, pl.id)
-    db.delete(pl)  # items cascade
-    db.commit()
+    # Not in the middle of a sync of it (`sync_claim`): a push that read the list before the delete
+    # and wrote YouTube after it planned from no items at all, so it emptied the YouTube playlist a
+    # local-only delete means to keep. The push checks for a vanished list too (`PlaylistGone`);
+    # this refuses the delete while a sync runs rather than race it.
+    with sync_claim(db, pl.id) as free:
+        if not free:
+            raise _refused(409, "sync_busy")
+        if on_youtube:
+            if not pl.yt_playlist_id:
+                raise _refused(400, "not_on_youtube")
+            if not has_write_scope(user):
+                raise _refused(403, "youtube_editing_off")
+            try:
+                with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_DELETE):
+                    with YouTubeClient(db, user) as yt:
+                        yt.delete_playlist(pl.yt_playlist_id)
+            except YouTubeError as exc:
+                raise youtube_refusal(exc)
+        # The playlist first, as every item write: the cascade deletes the items before the list row.
+        lock_playlists(db, pl.id)
+        db.delete(pl)  # items cascade
+        db.commit()
     return {"deleted": playlist_id}