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, 11 insertions, 0 deletions

diff --git a/src/purgatory/sync/mod.rs b/src/purgatory/sync/mod.rs
new file mode 100644
index 0000000..7b6d64a
--- /dev/null
+++ b/src/purgatory/sync/mod.rs
@@ -0,0 +1,11 @@
+//! Purgatory sync module for background git data synchronization.
+//!
+//! This module implements identifier-based syncing with:
+//! - Batched OID fetching across all purgatory events for an identifier
+//! - Domain-based throttling (configurable requests/minute per domain)
+//! - Exponential backoff per identifier (20s → 2m, then 2m intervals)
+//! - Debouncing for burst event arrivals
+
+mod queue;
+
+pub use queue::SyncQueueEntry;