feat(PROJ-40,PROJ-41): Prometheus Metriken + Dashboard Zeitreihe

- PROJ-40: /api/health mit Version+Uptime, /metrics Prometheus-Format
  (mails_last_60min/24h/7d/30d, mails_total, storage_bytes, tenants_total,
   users_total, uptime_seconds) — Token-Schutz optional konfigurierbar
- PROJ-41: GET /api/admin/stats/timeseries (30-Tage tagesgenau, Tenant-scoped)
  + SVG-Balkendiagramm im Dashboard (Mail-Eingang letzte 30 Tage)
- storage.DBQueryRow() Helper für Metrics-Queries ohne Pool-Exposition
- config.MetricsConfig (enabled, token) in config.go

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/storage/storage.go

@@ -1280,3 +1280,18 @@ func (s *Store) GetVerifyStatus(ctx context.Context, id string) (VerifyStatus, e
 	vs.VerifiedAt = at
 	return vs, nil
 }
+
+// DBQueryRow exposes a single DB query row for use by the API metrics handler.
+// Returns a no-op row if no DB is configured.
+func (s *Store) DBQueryRow(ctx context.Context, sql string, args ...interface{}) interface {
+	Scan(dest ...interface{}) error
+} {
+	if s.db == nil {
+		return &noopRow{}
+	}
+	return s.db.QueryRow(ctx, sql, args...)
+}
+
+type noopRow struct{}
+
+func (n *noopRow) Scan(dest ...interface{}) error { return nil }