fix: write heartbeat file at startup to avoid cold-start unhealthy window

scripts/scheduler.ts

@@ -731,6 +731,7 @@ async function main(): Promise<void> {
 
   validateEnvironment();
   ensureLogsDir();
+  fs.writeFileSync(path.join(LOGS_DIR, 'scheduler.heartbeat'), new Date().toISOString());
 
   // Crash recovery: mark orphaned jobs as failed
   await recoverFromCrash();