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.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/purgatory/mod.rs b/src/purgatory/mod.rs
index 88377fb..34a8e7a 100644
--- a/src/purgatory/mod.rs
+++ b/src/purgatory/mod.rs
@@ -12,6 +12,7 @@
 //! - **Separate stores**: State events and PR events use different indexing strategies
 
 mod helpers;
+pub mod sync;
 mod types;
 
 use anyhow::{bail, Result};