feat(sync): add ConnectedDegraded status for failed historic sync

- Add ConnectionStatus::ConnectedDegraded (status=4 in metrics)
- Track batch failures via PendingBatch.failed field
- Track relay-level failures via RelayState.historic_sync_had_failures
- Transition to ConnectedDegraded when any batch fails during historic sync
- Add is_live_sync_active() helper for cleaner match patterns
- Update state machine diagram with ConnectedDegraded transitions
- Update metrics docs with status=4 and example queries

Fixes issue where relays with failed negentropy retries would
incorrectly transition to Connected status despite missing data.
Now operators can distinguish 'fully synced' vs 'degraded (partial data)'.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/sync/algorithms.rs b/src/sync/algorithms.rs
index e083dc8..7536f41 100644
--- a/src/sync/algorithms.rs
+++ b/src/sync/algorithms.rs
@@ -405,6 +405,7 @@ mod tests {
                 requested_event_ids: None,
                 received_event_ids: None,
                 retry_count: 0,
+                failed: false,
             }],
         );
 
@@ -520,6 +521,7 @@ mod tests {
                 requested_event_ids: None,
                 received_event_ids: None,
                 retry_count: 0,
+                failed: false,
             }],
         );