Persist dashboard backup metrics separately

internal/store/store.go

@@ -70,8 +70,32 @@ func (s *Store) SaveRuns(runs []model.Run) error {
 	return writeJSONAtomic(s.runsPath(), runs, 0600)
 }
 
+func (s *Store) LoadBackupMetrics() ([]model.BackupMetric, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	var metrics []model.BackupMetric
+	if err := readJSON(s.backupMetricsPath(), &metrics); errors.Is(err, os.ErrNotExist) {
+		return []model.BackupMetric{}, nil
+	} else if err != nil {
+		return nil, err
+	}
+	return metrics, nil
+}
+
+func (s *Store) SaveBackupMetrics(metrics []model.BackupMetric) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if len(metrics) > 5000 {
+		metrics = metrics[len(metrics)-5000:]
+	}
+	return writeJSONAtomic(s.backupMetricsPath(), metrics, 0600)
+}
+
 func (s *Store) configPath() string { return filepath.Join(s.dir, "config.json") }
 func (s *Store) runsPath() string   { return filepath.Join(s.dir, "runs.json") }
+func (s *Store) backupMetricsPath() string {
+	return filepath.Join(s.dir, "backup-metrics.json")
+}
 
 func readJSON(path string, target any) error {
 	b, err := os.ReadFile(path)

internal/store/store_test.go

@@ -3,6 +3,7 @@ package store
 import (
 	"os"
 	"testing"
+	"time"
 
 	"git.casaderoll.de/michael/urbm/internal/model"
 )
@@ -28,3 +29,32 @@ func TestStoreInitializesAndPersists(t *testing.T) {
 		t.Fatalf("config mode = %o", info.Mode().Perm())
 	}
 }
+
+func TestBackupMetricsPersistSeparatelyFromRuns(t *testing.T) {
+	s := New(t.TempDir())
+	if err := s.Init(); err != nil {
+		t.Fatal(err)
+	}
+	finished := time.Date(2026, 7, 13, 1, 30, 0, 0, time.UTC)
+	metrics := []model.BackupMetric{{SchemaVersion: model.SchemaVersion, RunID: "run-1", JobID: "job-1", FinishedAt: finished, Status: "success", BytesProcessed: 42, FilesProcessed: 7}}
+	if err := s.SaveBackupMetrics(metrics); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.SaveRuns(nil); err != nil {
+		t.Fatal(err)
+	}
+	loaded, err := s.LoadBackupMetrics()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(loaded) != 1 || loaded[0].RunID != "run-1" || loaded[0].BytesProcessed != 42 {
+		t.Fatalf("backup metrics = %#v", loaded)
+	}
+	info, err := os.Stat(s.backupMetricsPath())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if info.Mode().Perm() != 0600 {
+		t.Fatalf("backup metrics mode = %o", info.Mode().Perm())
+	}
+}