fix(server): drain scheduler on unexpected serve error before exit

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/server/main.go

@@ -185,6 +185,12 @@ func main() {
 		log.Printf("server stopped")
 	case err := <-serveErr:
 		if err != nil {
+			// Unwind scheduler.Run via ctx.Done and wait for the in-flight
+			// RunOnce to finish before exiting, so an unexpected serve error
+			// mid-run doesn't kill the process during a check/notify write —
+			// same hazard the ctx.Done branch above already guards against.
+			stop()
+			wg.Wait()
 			log.Fatalf("server: %v", err)
 		}
 	}