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, 3 insertions, 0 deletions

diff --git a/Cargo.toml b/Cargo.toml
index eceebcb..82c2de7 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -55,6 +55,9 @@ clap = { version = "4.5", features = ["derive", "env"] }
 anyhow = "1.0"
 thiserror = "1.0"
 
+# Async traits
+async-trait = "0.1"
+
 # Git (for future use)
 # git-http-backend = "0.3"