## What changed
- Add a storage-neutral `QueueStore` interface backed by a dedicated SQLite
database for ordered, thread-scoped user submissions.
- Support enqueueing, paginated listing, editing, deletion, and atomic
reordering, with a limit of 100 pending items per thread.
- Remove queued submissions when their thread is deleted.
## Testing
- Cover concurrent inserts, ordering and pagination, edits, deletion,
cross-thread isolation, queue limits, and thread cleanup.
GitOrigin-RevId: 0a8e95ac71137afc6f3a039d4b5fc8603110c891
