Add SyncQueueEntry with exponential backoff for purgatory sync

Implement the sync queue entry struct that tracks sync state per identifier:
- next_attempt: when the next sync should be attempted
- attempt_count: for backoff calculation (resets on new events)
- in_progress: prevents concurrent syncs for same identifier

Backoff schedule: 20s → 40s → 80s → 120s (capped at 2 minutes)

This is the foundation for the identifier-based purgatory sync system
that will replace the current per-event syncing approach.

0 files changed, 0 insertions, 0 deletions