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)