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.

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};

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;

src/purgatory/sync/queue.rs

@@ -0,0 +1,206 @@
+//! Sync queue entry for tracking sync state per identifier.
+
+use std::time::{Duration, Instant};
+
+/// Entry in the sync queue tracking when/how to sync an identifier.
+///
+/// Each identifier in purgatory has at most one `SyncQueueEntry` that tracks:
+/// - When the next sync attempt should occur
+/// - How many attempts have been made (for backoff calculation)
+/// - Whether a sync is currently in progress
+#[derive(Debug, Clone)]
+pub struct SyncQueueEntry {
+    /// Don't attempt sync before this time
+    pub next_attempt: Instant,
+
+    /// Number of sync attempts (for backoff calculation).
+    /// Reset to 0 when new event arrives for this identifier.
+    pub attempt_count: u32,
+
+    /// Whether a sync is currently in progress for this identifier.
+    /// Prevents concurrent sync operations for the same identifier.
+    pub in_progress: bool,
+}
+
+impl SyncQueueEntry {
+    /// Create a new sync queue entry with the given initial delay.
+    ///
+    /// # Arguments
+    /// * `delay` - How long to wait before the first sync attempt
+    pub fn new(delay: Duration) -> Self {
+        Self {
+            next_attempt: Instant::now() + delay,
+            attempt_count: 0,
+            in_progress: false,
+        }
+    }
+
+    /// Calculate backoff duration based on attempt count.
+    ///
+    /// Backoff schedule:
+    /// - Attempt 1: 20s
+    /// - Attempt 2: 40s
+    /// - Attempt 3: 80s
+    /// - Attempt 4+: 120s (capped at 2 minutes)
+    ///
+    /// The formula is: min(20s * 2^(attempt_count-1), 120s)
+    pub fn backoff(&self) -> Duration {
+        if self.attempt_count == 0 {
+            return Duration::from_secs(20);
+        }
+
+        let base = Duration::from_secs(20);
+        let exponent = self.attempt_count.saturating_sub(1).min(3);
+        let multiplier = 2u32.saturating_pow(exponent);
+        (base * multiplier).min(Duration::from_secs(120))
+    }
+
+    /// Check if this entry is ready for a sync attempt.
+    ///
+    /// Returns true if:
+    /// - No sync is currently in progress
+    /// - The next_attempt time has passed
+    pub fn is_ready(&self) -> bool {
+        !self.in_progress && Instant::now() >= self.next_attempt
+    }
+
+    /// Called when a new event arrives for this identifier.
+    ///
+    /// Resets the attempt count to 0 (fresh start) and updates
+    /// next_attempt if the new delay would be sooner.
+    ///
+    /// # Arguments
+    /// * `delay` - The delay for the new event
+    pub fn on_new_event(&mut self, delay: Duration) {
+        self.attempt_count = 0;
+        let new_attempt = Instant::now() + delay;
+        if new_attempt < self.next_attempt {
+            self.next_attempt = new_attempt;
+        }
+    }
+
+    /// Called when a sync attempt completes (successfully or not).
+    ///
+    /// Marks the entry as not in progress, increments the attempt count,
+    /// and schedules the next attempt based on backoff.
+    ///
+    /// Only updates timing if the current next_attempt has passed
+    /// (prevents double-scheduling if called multiple times).
+    pub fn on_sync_complete(&mut self) {
+        self.in_progress = false;
+        if self.next_attempt <= Instant::now() {
+            self.attempt_count += 1;
+            self.next_attempt = Instant::now() + self.backoff();
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn backoff_doubles_up_to_cap() {
+        // Test that backoff follows: 20s → 40s → 80s → 120s → 120s (capped)
+        let mut entry = SyncQueueEntry::new(Duration::from_secs(0));
+
+        // Attempt 0 (initial state): 20s
+        assert_eq!(entry.backoff(), Duration::from_secs(20));
+
+        // Simulate completing attempts and check backoff
+        entry.attempt_count = 1;
+        assert_eq!(entry.backoff(), Duration::from_secs(20)); // 20 * 2^0 = 20
+
+        entry.attempt_count = 2;
+        assert_eq!(entry.backoff(), Duration::from_secs(40)); // 20 * 2^1 = 40
+
+        entry.attempt_count = 3;
+        assert_eq!(entry.backoff(), Duration::from_secs(80)); // 20 * 2^2 = 80
+
+        entry.attempt_count = 4;
+        assert_eq!(entry.backoff(), Duration::from_secs(120)); // 20 * 2^3 = 160, capped to 120
+
+        entry.attempt_count = 5;
+        assert_eq!(entry.backoff(), Duration::from_secs(120)); // Still capped
+
+        entry.attempt_count = 100;
+        assert_eq!(entry.backoff(), Duration::from_secs(120)); // Always capped
+    }
+
+    #[test]
+    fn new_event_resets_attempt_count() {
+        let mut entry = SyncQueueEntry::new(Duration::from_secs(60));
+
+        // Simulate several sync attempts
+        entry.attempt_count = 5;
+        entry.next_attempt = Instant::now() + Duration::from_secs(120);
+
+        // New event arrives with shorter delay
+        entry.on_new_event(Duration::from_secs(10));
+
+        // Attempt count should be reset
+        assert_eq!(entry.attempt_count, 0);
+
+        // next_attempt should be updated to the sooner time
+        // (within a small tolerance for test timing)
+        let expected = Instant::now() + Duration::from_secs(10);
+        assert!(entry.next_attempt <= expected + Duration::from_millis(100));
+        assert!(entry.next_attempt >= expected - Duration::from_millis(100));
+    }
+
+    #[test]
+    fn new_event_does_not_delay_if_already_sooner() {
+        let mut entry = SyncQueueEntry::new(Duration::from_secs(5));
+        let original_next = entry.next_attempt;
+
+        // New event arrives with longer delay - should not push back
+        entry.on_new_event(Duration::from_secs(60));
+
+        // Attempt count should still be reset
+        assert_eq!(entry.attempt_count, 0);
+
+        // But next_attempt should not be pushed back
+        assert!(entry.next_attempt <= original_next + Duration::from_millis(100));
+    }
+
+    #[test]
+    fn is_ready_checks_both_conditions() {
+        let mut entry = SyncQueueEntry::new(Duration::from_secs(0));
+
+        // Should be ready initially (no delay, not in progress)
+        // Note: there might be a tiny delay, so we wait a moment
+        std::thread::sleep(Duration::from_millis(10));
+        assert!(entry.is_ready());
+
+        // Mark as in progress - should not be ready
+        entry.in_progress = true;
+        assert!(!entry.is_ready());
+
+        // Not in progress but future next_attempt - should not be ready
+        entry.in_progress = false;
+        entry.next_attempt = Instant::now() + Duration::from_secs(60);
+        assert!(!entry.is_ready());
+    }
+
+    #[test]
+    fn on_sync_complete_increments_and_schedules() {
+        let mut entry = SyncQueueEntry::new(Duration::from_secs(0));
+        std::thread::sleep(Duration::from_millis(10)); // Ensure next_attempt has passed
+
+        entry.in_progress = true;
+        entry.attempt_count = 0;
+
+        entry.on_sync_complete();
+
+        // Should no longer be in progress
+        assert!(!entry.in_progress);
+
+        // Attempt count should be incremented
+        assert_eq!(entry.attempt_count, 1);
+
+        // Next attempt should be scheduled with backoff (20s for attempt 1)
+        let expected = Instant::now() + Duration::from_secs(20);
+        assert!(entry.next_attempt >= expected - Duration::from_millis(100));
+        assert!(entry.next_attempt <= expected + Duration::from_millis(100));
+    }
+}