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.

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