[Feature Request] Lock queue command

**Is your feature request related to a problem? Please describe.**
A DJ (or higher) should be able to lock the queue to prevent any new songs from being added. Existing songs in the queue continue to play. DJ (and higher) are able to bypass the lock and add new songs.

Up for debate: Queue unlocks automatically once the queue is empty.

**Describe the solution you'd like**
1. The lock command should behave like a toggle to allow/prevent songs from being added.
2. Should continue to re-add existing songs if repeat is toggled on.

**Have you confirmed that this idea is not already described here?**
Not an existing feature nor has it already been described.

Start by locating the queue command, the logic that accepts new songs, and the repeat behavior described in the issue. Trace the existing DJ-or-higher permission checks before deciding where the toggle belongs. Done means lower-ranked users are blocked while existing and repeating songs continue, and authorized users can bypass the lock.

Written by the indexing model from the issue text.