Reduce metric writes and preserve scheduler history

internal/store/store_test.go

@@ -43,10 +43,13 @@ func TestBackupMetricsPersistSeparatelyFromRuns(t *testing.T) {
 	if err := s.SaveRuns(nil); err != nil {
 		t.Fatal(err)
 	}
-	loaded, err := s.LoadBackupMetrics()
+	loaded, legacy, err := s.LoadBackupMetrics()
 	if err != nil {
 		t.Fatal(err)
 	}
+	if legacy {
+		t.Fatal("compacted metric file was detected as legacy JSON array")
+	}
 	if len(loaded) != 1 || loaded[0].RunID != "run-1" || loaded[0].BytesProcessed != 42 {
 		t.Fatalf("backup metrics = %#v", loaded)
 	}
@@ -58,3 +61,32 @@ func TestBackupMetricsPersistSeparatelyFromRuns(t *testing.T) {
 		t.Fatalf("backup metrics mode = %o", info.Mode().Perm())
 	}
 }
+
+func TestBackupMetricsMigrateFromJSONArrayAndAppend(t *testing.T) {
+	s := New(t.TempDir())
+	finished := time.Date(2026, 7, 13, 1, 30, 0, 0, time.UTC)
+	legacyJSON := `[{"schemaVersion":1,"runId":"old","jobId":"job-1","finishedAt":"2026-07-13T01:30:00Z","status":"success","bytesProcessed":42,"filesProcessed":7}]`
+	if err := os.WriteFile(s.legacyBackupMetricsPath(), []byte(legacyJSON), 0600); err != nil {
+		t.Fatal(err)
+	}
+	metrics, legacy, err := s.LoadBackupMetrics()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !legacy || len(metrics) != 1 || metrics[0].RunID != "old" {
+		t.Fatalf("legacy metrics = %#v, legacy = %v", metrics, legacy)
+	}
+	if err := s.SaveBackupMetrics(metrics); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.AppendBackupMetric(model.BackupMetric{SchemaVersion: model.SchemaVersion, RunID: "new", JobID: "job-1", FinishedAt: finished.Add(time.Hour), Status: "success"}); err != nil {
+		t.Fatal(err)
+	}
+	metrics, legacy, err = s.LoadBackupMetrics()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if legacy || len(metrics) != 2 || metrics[1].RunID != "new" {
+		t.Fatalf("appended metrics = %#v, legacy = %v", metrics, legacy)
+	}
+}