Add SyncContext trait and MockSyncContext for purgatory sync

Implement the abstraction layer for purgatory sync operations:

- SyncContext trait: defines interface for repository data fetching,
  OID existence checks, git fetch operations, and event processing
- ProcessResult: captures outcomes when releasing events from purgatory
- MockSyncContext: test mock with builder pattern for configuring:
  - Clone URLs and which OIDs each URL provides
  - Needed OIDs (simulates purgatory state)
  - URL failure simulation
  - Fetch logging for assertions

The trait uses async_trait for async method support and requires
Send + Sync for use in concurrent sync operations.

This abstraction enables unit testing of sync logic without I/O,
while the real implementation (to be added later) will connect
to actual database, git, and relay systems.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/purgatory/sync/mod.rs b/src/purgatory/sync/mod.rs
index b29f10e..1ac0cb1 100644
--- a/src/purgatory/sync/mod.rs
+++ b/src/purgatory/sync/mod.rs
@@ -6,8 +6,13 @@
 //! - Exponential backoff per identifier (20s → 2m, then 2m intervals)
 //! - Debouncing for burst event arrivals
 
+mod context;
 mod queue;
 mod throttle;
 
+pub use context::{ProcessResult, SyncContext};
 pub use queue::SyncQueueEntry;
 pub use throttle::{DomainThrottle, ThrottleManager};
+
+#[cfg(test)]
+pub use context::mock::MockSyncContext;