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_youtube_refusals.py

@@ -37,7 +37,7 @@ def test_a_failure_is_sorted_by_what_the_user_can_do(exc, reason):
     ("route", "target"),
     [
         ("/api/sync/subscriptions", "app.routes.sync.import_subscriptions"),
-        ("/api/playlists/sync-youtube", "app.routes.playlists.sync_user_playlists"),
+        ("/api/playlists/sync-youtube", "app.routes.playlists.pull_user_playlists"),
     ],
 )
 def test_a_manual_sync_youtube_refuses_is_a_422_not_a_500(