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/system.ts

@@ -110,6 +110,11 @@ export interface SystemStats {
   estimate: SystemStatsEstimate;
 }
 
+export interface TimeseriesPoint {
+  day: string;  // "2026-04-05"
+  count: number;
+}
+
 export interface SecurityCheck {
   name: string;
   status: "ok" | "warning" | "error";
@@ -164,6 +169,10 @@ export async function getSystemStats(): Promise<SystemStats> {
   return request<SystemStats>("/api/admin/system/stats");
 }
 
+export async function getMailTimeseries(days = 30): Promise<{ days: number; points: TimeseriesPoint[] }> {
+  return request<{ days: number; points: TimeseriesPoint[] }>(`/api/admin/stats/timeseries?days=${days}`);
+}
+
 // ── Services ──────────────────────────────────────────────────────────────────
 
 export async function getServices(): Promise<ServiceStatus[]> {