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/tests/test_abuse_limits.py

@@ -100,7 +100,7 @@ def test_the_single_playlist_repull_shares_the_pull_cooldown(db, as_user, make_u
     db.commit()
     monkeypatch.setattr(plroutes, "has_read_scope", lambda u: True)
     monkeypatch.setattr(plroutes, "repull_playlist", lambda d, u, p: {"ok": True})
-    monkeypatch.setattr(plroutes, "sync_user_playlists", lambda d, u: {"ok": True})
+    monkeypatch.setattr(plroutes, "pull_user_playlists", lambda d, u: {"ok": True})
 
     c = as_user(user)
     codes = [c.post(f"/api/playlists/{pl.id}/revert-youtube").status_code for _ in range(4)]