Keep manual audio and subtitle choices within the current programme

[mechaigor-hash]

Summary

Follow-up to #641. This keeps remote-control audio/subtitle selections local to the currently playing scheduled programme block, so a viewer's manual choice survives bumpers/commercials and is cleared when the next programme starts.

Behaviour

• remembers AUDIO / SUBS button choices only while a feature video is playing
• reapplies the remembered selection if the same programme block resumes after interstitials
• clears remembered selections when playback enters a new programme block
• does not persist choices to disk/config
• does not attempt language detection
• does not apply the programme choice to commercials/bumpers

Notes

This is intentionally marked as a draft because it is stacked on top of #641. Until #641 is merged, GitHub will show both the remote-button commit and this follow-up commit in this PR.

Testing

python3 -m py_compile fs42/station_player.py fs42/fs42_server/api/player.py
python3 -m pytest test/test_station_player_track_preferences.py test/test_nfo_agent.py -q

Result:

28 passed

[shane-mason]

Starting review today.