refactor: Get playback status and metadata from one subscription

[abusch]

Subscribes to each group's playbackExtended namespace (sinuous-client 0.2.2) instead of both its playback and playbackMetadata ones. That saves a subscription per group (25 instead of 30 with 5 groups), and the playback status and metadata always describe the same moment.

• One ExtendedPlaybackStatus event updates both; the metadata handling moved, unchanged, into Service::update_metadata.
• Failed subscriptions are still reported with the group's name.

Tried read-only against a real household with debug logs: only PlaybackExtended subscriptions were made, without warnings; the current track, source, position and next track showed on start-up; and toggling shuffle on an idle speaker (then back) showed each change as the speaker reported it.

Unverified: whether speakers send playbackError events to playbackExtended subscribers. The namespace isn't in Sonos's public docs. They are still handled if they arrive; if they don't, playback errors stop showing in the title bar, and subscribing to playback again for them would undo the saving.

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