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, 3 insertions, 1 deletions

diff --git a/src/sync/metrics.rs b/src/sync/metrics.rs
index db7dd20..0f56911 100644
--- a/src/sync/metrics.rs
+++ b/src/sync/metrics.rs
@@ -53,7 +53,7 @@ impl SyncMetrics {
         let relay_connected = IntGaugeVec::new(
             Opts::new(
                 "ngit_sync_relay_connected",
-                "Relay connection status (0=disconnected, 1=connecting, 2=syncing, 3=connected)",
+                "Relay connection status (0=disconnected, 1=connecting, 2=syncing, 3=connected, 4=connected_degraded)",
             ),
             &["relay"],
         )?;
@@ -208,6 +208,7 @@ impl SyncMetrics {
     /// - Connecting = 1 (connection attempt in progress)
     /// - Syncing = 2 (connected, historic sync in progress)
     /// - Connected = 3 (connected, historic sync complete)
+    /// - ConnectedDegraded = 4 (connected, historic sync failed but live sync active)
     ///
     /// This is separate from health state and provides more granular connection lifecycle tracking.
     ///
@@ -222,6 +223,7 @@ impl SyncMetrics {
             ConnectionStatus::Connecting => 1,
             ConnectionStatus::Syncing => 2,
             ConnectionStatus::Connected => 3,
+            ConnectionStatus::ConnectedDegraded => 4,
         };
         self.relay_connected
             .with_label_values(&[relay])