feat(PROJ-34): Retention-Tab + pro-Mandant Aufbewahrungsfristen

- tenantstore: retention_days Spalte, GetRetentionDays/SetRetentionDays
- storage.Save(): per-tenant retention überschreibt globale config
- API: GET /api/admin/retention, PUT /api/admin/tenant/{id}/retention
- Frontend: RetentionTab mit globaler Policy-Anzeige, Mandanten-Tabelle,
  Bearbeiten-Dialog und Purge-Button (superadmin only)

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

src/app/admin/page.tsx

@@ -76,6 +76,7 @@ import { LabelsTab } from "@/components/admin/tabs/LabelsTab";
 import { CertTab } from "@/components/admin/tabs/CertTab";
 import { ModulesTab } from "@/components/admin/ModulesTab";
 import { IMAPSettingsTab } from "@/components/admin/tabs/IMAPSettingsTab";
+import { RetentionTab } from "@/components/admin/tabs/RetentionTab";
 import { ResetPasswordDialog, DeleteUserDialog } from "@/components/admin/UserDialogs";
 
 const AUDIT_PAGE_SIZE = 25;
@@ -807,6 +808,7 @@ export default function AdminPage() {
             {isSuperAdmin && <TabsTrigger value="security">Security</TabsTrigger>}
             {isSuperAdmin && <TabsTrigger value="cert" onClick={loadCert}>Zertifikat</TabsTrigger>}
             {isSuperAdmin && <TabsTrigger value="tenants" onClick={loadTenants}>Mandanten</TabsTrigger>}
+            {isSuperAdmin && <TabsTrigger value="retention">Retention</TabsTrigger>}
             {isSuperAdmin && <TabsTrigger value="modules">Module</TabsTrigger>}
           </TabsList>
 
@@ -1085,6 +1087,12 @@ export default function AdminPage() {
             </TabsContent>
           )}
 
+          {isSuperAdmin && (
+            <TabsContent value="retention">
+              <RetentionTab />
+            </TabsContent>
+          )}
+
           <TabsContent value="modules" className="mt-4">
             <ModulesTab />
           </TabsContent>