fix(saas): SQL type cast fixes for E2E relay flow

- key_pool.rs: cast cooldown_until to timestamptz for comparison with NOW()
- key_pool.rs: cast request_count to bigint (INT4→INT8) for sqlx decoding
- service.rs: cast cooldown_until to timestamptz in quota sort query
- scheduler.rs: cast last_seen_at to timestamptz in device cleanup
- totp.rs: use DateTime<Utc> instead of rfc3339 string for updated_at

crates/zclaw-saas/src/relay/service.rs

@@ -653,7 +653,7 @@ pub async fn sort_candidates_by_quota(
                 AND uw.window_minute = to_char(date_trunc('minute', NOW()), 'YYYY-MM-DDTHH24:MI')
             WHERE pk.provider_id = ANY($1)
               AND pk.is_active = TRUE
-              AND (pk.cooldown_until IS NULL OR pk.cooldown_until <= NOW())
+              AND (pk.cooldown_until IS NULL OR pk.cooldown_until::timestamptz <= NOW())
             GROUP BY pk.provider_id
             "#,
         )