fix: fallback to default arbiter_service_id in explicit channel leases

Explicit leases created by failover didn't include arbiter_service_id,
causing arbiter verdict to never be saved (pairedExecutionContext was
undefined). Now falls back to ARBITER_SERVICE_ID from config when the
stored lease has null arbiter_service_id.

src/service-routing.ts

@@ -45,7 +45,9 @@ function normalizeLeaseRow(
       : null,
     arbiter_service_id: row.arbiter_service_id
       ? normalizeServiceId(row.arbiter_service_id)
-      : null,
+      : isArbiterEnabled()
+        ? ARBITER_SERVICE_ID
+        : null,
     activated_at: row.activated_at,
     reason: row.reason,
     explicit,