fix(api): a waiting reorder deals from the fresh order; the commit check sees staged writes

Round 2 of the deep review, on round 1's own fixes:
- `ORDER BY position … FOR UPDATE` sorts before it locks, so the reorder that waited got the
  fresh rows in its STALE order and dealt the pins it left out (a tab with Plex off) back to their
  old slots. It locks in id order — one global order, so two reorders cannot deadlock — and sorts
  the fresh rows itself. The race test now sends that partial list and fails unless tab B really
  blocked.
- The request check saw only writes the handler flushed: a `db.delete` left staged, or a flush
  followed by a released savepoint, passed. Staged changes count at the end of the request, and
  only the ROOT transaction's end clears it (a savepoint test drives the dependency directly).
- The `ok` test exempted every call as a serializer, hiding four domain REPORTS (YouTube sync,
  revert, push; Plex sync) without `ok`. They are spread into `{"ok": True, …}`; the exemption is a
  named list of real serializers.

backend/app/routes/playlists.py

@@ -324,7 +324,7 @@ def sync_youtube(
     _check_pull_cooldown(user)
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
-            return sync_user_playlists(db, user)
+            return {"ok": True, **sync_user_playlists(db, user)}
     except YouTubeError as exc:
         db.rollback()
         raise youtube_refusal(exc)
@@ -347,7 +347,7 @@ def revert_youtube(
     _check_pull_cooldown(user)
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
-            return repull_playlist(db, user, pl)
+            return {"ok": True, **repull_playlist(db, user, pl)}
     except YouTubeError as exc:
         db.rollback()
         raise youtube_refusal(exc)
@@ -407,7 +407,7 @@ def push(
             plan = plan_merge(db, user, pl, live=live)
             if plan["units_estimate"] > quota.remaining_today(db):
                 raise _refused(429, "quota_exceeded")
-            return merge_playlist(db, user, pl, live=live)
+            return {"ok": True, **merge_playlist(db, user, pl, live=live)}
     except YouTubeError as exc:
         db.rollback()
         raise youtube_refusal(exc)