fix(playlists): item writes lock the list first; one bulk pull per user; the claim on its own connection

- Every playlist item write (add, remove, move, reorder, rename; Plex remove and remove-bulk) row-locks
  the playlist before it touches an item, a move both lists in id order: remove beside reorder was a
  deadlock, and two adds of one video a unique-key 500.
- The scheduled mirror refresh re-reads dirty under that lock, so an edit made during its YouTube calls
  is not replaced by YouTube's order.
- A user's bulk pull holds a per-user advisory claim: two at once created duplicate mirrors. The manual
  sync answers 409 sync_all_busy instead of '0 synced', and like the re-pull spends its cooldown only
  on a pull that runs.
- Deleted mirrors are committed at once, not only when a later list commits.
- The claims open an unpooled connection of their own instead of a third pooled one per push.

backend/app/routes/plex/playlists.py

@@ -252,7 +252,9 @@ def playlist_remove_item(
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist")
+    # Locked like the adds and the reorder: the list row first, then its items — a remove that
+    # locked an item row before a reorder locked the list deadlocked against it.
+    pl = owned_or_404(db, PlexPlaylist, pid, user, what="playlist", lock=True)
     it = by_rating_key(db, PlexItem, item_rating_key)
     if it is not None:
         db.execute(
@@ -399,7 +401,7 @@ def playlist_remove_bulk(
 ) -> dict:
     """Remove many items at once (a whole season/show, or a multi-select). Returns how many were
     removed + 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)
     ids = [it.id for it in _items_by_rks(db, body.item_rating_keys)]
     removed = 0
     if ids: