fix(web): suspend hidden metadata polling

[pitill0]

Summary

• suspend Web metadata polling while the document is hidden;
• discard stale metadata responses after a visibility transition;
• refresh metadata immediately when active playback returns to the foreground;
• prevent overlapping requests and polling timers;
• keep audio playback and network recovery independent from metadata visibility;
• update metadata lifecycle documentation and the Unreleased changelog.

Closes #211.

Validation

• make gate
• 893 tests passed
• focused metadata visibility and JavaScript tests passed
• manual mobile verification will follow after deployment

Manual verification

• Start playback and confirm normal metadata updates.
• Lock the mobile device or switch to another application.
• Confirm that /api/metadata requests stop while FluxTuner is hidden.
• Return to FluxTuner and confirm an immediate metadata refresh.
• Confirm that audio continues playing throughout the visibility transition.
• Confirm that paused or stopped playback does not restart metadata polling.