Fix strict TypeScript checks for server

plugin/core/persistence.ts

@@ -79,6 +79,9 @@ export interface ClientRecord {
   /** Last successful authentication timestamp (UTC unix seconds) */
   lastAuthenticatedAt?: number;
 
+  /** Last successful pairing timestamp (UTC unix seconds) */
+  pairedAt?: number;
+
   /**
    * Recent nonces used in authentication attempts.
    * This is a rolling window that may be cleared on restart.
@@ -151,6 +154,7 @@ export interface SerializedClientRecord {
   status: ClientLivenessStatus;
   lastHeartbeatAt?: number;
   lastAuthenticatedAt?: number;
+  pairedAt?: number;
   createdAt: number;
   updatedAt: number;
   // Note: recentNonces and recentHandshakeAttempts are intentionally
@@ -203,6 +207,7 @@ export function serializeClientRecord(record: ClientRecord): SerializedClientRec
     status: record.status,
     lastHeartbeatAt: record.lastHeartbeatAt,
     lastAuthenticatedAt: record.lastAuthenticatedAt,
+    pairedAt: record.pairedAt,
     createdAt: record.createdAt,
     updatedAt: record.updatedAt
   };