Stream sync improvements:
- Server now sends changed_at timestamp + raw remaining seconds
- Client schedules UI updates based on changed_at + measured buffer lag
- Removed server-side delay logic entirely
- Poll interval set to 10s (was 15s, briefly 5s which caused issues)
Connection exhaustion fix:
- Added in-flight guards to update-mini-now-playing and poll-now-playing
- Prevents fetch pileup when server is slow or stalled
- Each poller skips if previous request hasn't completed
Other:
- Include changed_at in now-playing JSON API response
- Replace em dashes with hyphens throughout
- Update cl-streamer submodule (get-metadata-changed-at export)
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