Multi-server: most recent request wins (#252)

[rowkav09]

Part of #252. Rowan's rule (24 Sept): when more than one server is playing, the most recent request wins.

• Each server records when it last started something: playback began, resumed from pause, or moved to a different item.
• Playing beats paused; among those, the newest start wins.
• Nothing playing or paused anywhere: the first server's result shows, as before (a one-server setup is unchanged, and a failing first server still fails like before).
• A playing server still shows when the first server is down.
• Per-server backoff and the status-page rows are untouched.

Tests in test/multi-server.test.js.

[github-actions]

/mira pause

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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