MPRIS only updated on track change

### What Happened?

If track metadata changes (such as loading album art), MPRIS isn't updated with the new data

### Steps to Reproduce

1. Open Music and add some tracks
2. Without changing track, open MPRIS
3. See that track metadata in the app is complete while in MPRIS you may be missing artist or have file URI instead of track name

### Expected Behavior

We should update MPRIS metadata when it changes in the audio object. Perhaps the metadata discoverer should fire a signal since metadata has to all be updated at once in MPRIS?

### OS Version

8.x (Circe)

### Session Type

Secure Session (Wayland)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_

Start by tracing how the audio object and metadata discoverer communicate with the MPRIS integration, focusing on when updates are emitted after metadata changes without a track change. Done means MPRIS reflects complete artist, album art, and track URI or name metadata while the same track continues playing.

Written by the indexing model from the issue text.