Reduce metric writes and preserve scheduler history

internal/service/safety_test.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"git.casaderoll.de/michael/urbm/internal/model"
+	"git.casaderoll.de/michael/urbm/internal/queue"
 	"git.casaderoll.de/michael/urbm/internal/store"
 )
 
@@ -39,7 +40,8 @@ func TestRepositoryLockHonorsContextCancellation(t *testing.T) {
 }
 
 func TestBackupMetricsSurviveClearedRunSnapshot(t *testing.T) {
-	st := store.New(t.TempDir())
+	dir := t.TempDir()
+	st := store.New(dir)
 	if err := st.Init(); err != nil {
 		t.Fatal(err)
 	}
@@ -47,13 +49,36 @@ func TestBackupMetricsSurviveClearedRunSnapshot(t *testing.T) {
 	finished := time.Date(2026, 7, 13, 1, 30, 0, 0, time.UTC)
 	run := model.Run{ID: "run-1", JobID: "job-1", TaskType: "backup", Status: "success", FinishedAt: &finished, BytesProcessed: 1024, FilesProcessed: 12}
 	s.queuePersistence([]model.Run{run})
+	before, err := os.ReadFile(filepath.Join(dir, "backup-metrics.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
 	s.queuePersistence(nil)
+	after, err := os.ReadFile(filepath.Join(dir, "backup-metrics.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(after) != string(before) {
+		t.Fatal("clearing activities rewrote the backup metric file")
+	}
 	metrics := s.BackupMetrics()
 	if len(metrics) != 1 || metrics[0].RunID != run.ID || metrics[0].FilesProcessed != 12 {
 		t.Fatalf("backup metrics after clear = %#v", metrics)
 	}
 }
 
+func TestLastRunUsesDurableMetricAfterActivityClear(t *testing.T) {
+	created := time.Date(2026, 7, 13, 1, 0, 0, 0, time.UTC)
+	finished := created.Add(3 * time.Hour)
+	q := queue.New(nil, nil)
+	q.RestoreHistory([]model.Run{{ID: "run-1", JobID: "job-1", Status: "success", CreatedAt: created}})
+	s := &Service{queue: q, backupMetrics: []model.BackupMetric{{RunID: "run-1", JobID: "job-1", Status: "success", FinishedAt: finished}}}
+	q.ClearHistory()
+	if got := s.LastRun("job-1"); !got.Equal(finished) {
+		t.Fatalf("last run after activity clear = %v, want %v", got, finished)
+	}
+}
+
 func TestBackupMetricsOnlyCaptureCompletedSuccessfulBackupsOnce(t *testing.T) {
 	st := store.New(t.TempDir())
 	s := &Service{store: st}