feat(sync): prevent infinite retry loop in negentropy validation

Add retry protection to negentropy event validation:

- Track retry_count in PendingBatch (incremented on each retry attempt)
- Detect when retry makes zero progress (relay returns no requested events)
- Abort retry and complete batch with partial results when stuck
- Log error with full details when retry protection triggers

This prevents infinite loops when:
- Relay has bugs and returns wrong events for ID queries
- Relay is malicious and returns unrelated events
- Relay has eventual consistency issues
- Network corruption causes incorrect responses

The protection triggers when received_count == 0 on a retry (relay
returned nothing we asked for), indicating the relay will never
provide the missing events.

Future work: Track failed batches in Prometheus metrics
(sync_failed_batches_total) for monitoring and alerting.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/sync/algorithms.rs b/src/sync/algorithms.rs
index 4679986..e083dc8 100644
--- a/src/sync/algorithms.rs
+++ b/src/sync/algorithms.rs
@@ -404,6 +404,7 @@ mod tests {
                 pagination_state: HashMap::new(),
                 requested_event_ids: None,
                 received_event_ids: None,
+                retry_count: 0,
             }],
         );
 
@@ -518,6 +519,7 @@ mod tests {
                 pagination_state: HashMap::new(),
                 requested_event_ids: None,
                 received_event_ids: None,
+                retry_count: 0,
             }],
         );