Add background sync loop for purgatory identifier processing

Implement the main sync loop that runs in the background and processes
identifiers that are ready for git data synchronization:

- Runs every 1 second (hardcoded interval, not configurable)
- Finds all ready identifiers where !in_progress && next_attempt <= now
- Spawns parallel tasks for each ready identifier
- Each task calls sync_identifier to try fetching git data from remotes
- Applies backoff when sync completes but events remain in purgatory
- Removes identifiers from queue when sync completes or no events remain

The loop integrates with the existing sync infrastructure:
- Uses SyncContext trait for testability
- Uses ThrottleManager for domain-based rate limiting
- Uses sync_identifier for the actual fetch orchestration

This enables automatic background fetching of git data for events in
purgatory, complementing the existing push-triggered sync path.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/purgatory/sync/mod.rs b/src/purgatory/sync/mod.rs
index 8ac9216..be89130 100644
--- a/src/purgatory/sync/mod.rs
+++ b/src/purgatory/sync/mod.rs
@@ -5,9 +5,11 @@
 //! - Domain-based throttling (configurable requests/minute per domain)
 //! - Exponential backoff per identifier (20s → 2m, then 2m intervals)
 //! - Debouncing for burst event arrivals
+//! - Background sync loop processing ready identifiers every 1 second
 
 mod context;
 mod functions;
+mod r#loop;
 mod queue;
 mod throttle;