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.

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