feat: implement soft expiry and revival for purgatory announcements

Two-phase expiry for announcement purgatory entries:
- Phase 1 (initial 30min timeout): delete bare repo, set soft_expired=true,
  extend expiry by 24h so the event is retained for potential revival
- Phase 2 (24h extended timeout): fully remove from purgatory

Revival: extend_announcement_expiry() now recreates the bare git repo
when called on a soft-expired entry (triggered by state event or git auth),
clearing soft_expired and resetting the expiry window.

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