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/app/admin/page.tsx

@@ -17,6 +17,7 @@ import {
   getServices,
   serviceAction,
   getSystemStats,
+  getMailTimeseries,
   uploadMailFiles,
   getUploadProgress,
   getSecurityAudit,
@@ -39,6 +40,7 @@ import {
   type StorageStats,
   type ServiceStatus,
   type SystemStats,
+  type TimeseriesPoint,
   type UploadJob,
   type SecurityAuditResult,
   type Tenant,
@@ -82,6 +84,7 @@ export default function AdminPage() {
   const [smtpStatus, setSmtpStatus] = useState<SMTPStatus | null>(null);
   const [storageStats, setStorageStats] = useState<StorageStats | null>(null);
   const [systemStats, setSystemStats] = useState<SystemStats | null>(null);
+  const [timeseries, setTimeseries] = useState<TimeseriesPoint[]>([]);
   const [apiOnline, setApiOnline] = useState<boolean | null>(null);
   const [dashLoading, setDashLoading] = useState(true);
   const [dashRefreshed, setDashRefreshed] = useState<Date | null>(null);
@@ -248,16 +251,18 @@ export default function AdminPage() {
   const loadDashboard = useCallback(async () => {
     setDashLoading(true);
     try {
-      const [smtp, health, storage, sysStats] = await Promise.allSettled([
+      const [smtp, health, storage, sysStats, ts] = await Promise.allSettled([
         getSMTPStatus(),
         getHealth(),
         getStorageStats(),
         getSystemStats(),
+        getMailTimeseries(30),
       ]);
       setSmtpStatus(smtp.status === "fulfilled" ? smtp.value : null);
       setApiOnline(health.status === "fulfilled" && health.value.status === "ok");
       setStorageStats(storage.status === "fulfilled" ? storage.value : null);
       setSystemStats(sysStats.status === "fulfilled" ? sysStats.value : null);
+      setTimeseries(ts.status === "fulfilled" ? ts.value.points : []);
       setDashRefreshed(new Date());
     } finally {
       setDashLoading(false);
@@ -718,6 +723,7 @@ export default function AdminPage() {
               smtpStatus={smtpStatus}
               storageStats={storageStats}
               systemStats={systemStats}
+              timeseries={timeseries}
               apiOnline={apiOnline}
               dashLoading={dashLoading}
               dashRefreshed={dashRefreshed}