Stop the music when you power off a speaker (#6074)

# What does this implement/fix?

Powering a speaker off stopped the speaker but never stopped its queue,
so Music Assistant carried on in the background. With Spotify this was
visible in the Spotify app: it kept showing music playing on "Music
Assistant Playback" for another song or two after the speaker was
already off. Pressing stop always worked, because stop ends the queue
and powering off did not.

Powering off now ends the queue as well.

- Power off ends the player's own queue, exactly as pressing stop
already did
- Only the player's own queue is ended, so powering off cannot reach
into another player's playback
- Split the queue stop into a public command that checks permissions and
an internal one that does not, matching how play is already split, so an
internal power off of a group keeps working for users with restricted
player access

**Related issue (if applicable):**

- Reported on Discord

## Types of changes

- [x] Bugfix (non-breaking change which fixes an issue) — `bugfix`

## Checklist

- [x] The code change is tested and works locally.
- [x] `pre-commit run --all-files` passes.
- [x] `pytest` passes, and tests have been added/updated under `tests/`
where applicable.
- [ ] For changes to shared models, the companion PR in
`music-assistant/models` is linked.
- [ ] For changes affecting the UI, the companion PR in
`music-assistant/frontend` is linked.
- [x] I have read and complied with the project's [AI
Policy](https://github.com/music-assistant/.github/blob/main/AI_POLICY.md)
for any AI-assisted contributions.
- [ ] I have [raised a PR against the documentation
repository](https://github.com/music-assistant/music-assistant.io/blob/main/CONTRIBUTING.md)
targeting the main or beta branch as appropriate.