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>

src/lib/api/index.ts

@@ -133,6 +133,7 @@ export type {
   SystemStatsActivity,
   SystemStatsEstimate,
   SystemStats,
+  TimeseriesPoint,
   SecurityCheck,
   SecurityAuditResult,
   CertInfo,
@@ -144,6 +145,7 @@ export {
   getSMTPStatus,
   getStorageStats,
   getSystemStats,
+  getMailTimeseries,
   getServices,
   serviceAction,
   getAuditLog,