Postgres store: return token_expires_at as the ISO string the port promises

pg returns a Date for TIMESTAMPTZ; a byte-identity check on a kept credential
saw a Date where the port types a string.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/store/postgres.ts

@@ -184,7 +184,8 @@ function mapLinkedIdentityRow(row: LinkedIdentityRow): StoredLinkedIdentity {
   }
 
   if (row.token_expires_at) {
-    identity.tokenExpiresAt = row.token_expires_at;
+    // pg hands back a Date for TIMESTAMPTZ whatever the row type says; the port promises an ISO string.
+    identity.tokenExpiresAt = new Date(row.token_expires_at).toISOString();
   }
 
   return identity;