feat(PROJ-29): Tenant-Quotas & Usage-Limits

- DB: max_storage_bytes, max_users, max_emails per Tenant (NULL = unlimited)
- storage/quota.go: CheckQuota() mit 60s-Cache, ErrQuotaExceeded
- Save() prüft Quota vor dem Schreiben — Ablehnung bei Hard-Limit
- tenantstore/quota.go: SetQuota(), GetQuota(), GetUsage()
- API: GET/PUT /api/admin/tenant/{id}/quota, GET /api/admin/quotas
- QuotaTab: Usage-Balken (Speicher/Nutzer/Mails), Edit-Dialog, Warnung ab 80%
- InvalidateQuotaCache() nach Quota-Änderung für sofortige Wirkung

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

src/app/admin/page.tsx

@@ -77,6 +77,7 @@ 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 { QuotaTab } from "@/components/admin/tabs/QuotaTab";
 import { ResetPasswordDialog, DeleteUserDialog } from "@/components/admin/UserDialogs";
 
 const AUDIT_PAGE_SIZE = 25;
@@ -809,6 +810,7 @@ export default function AdminPage() {
             {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="quotas">Quotas</TabsTrigger>}
             {isSuperAdmin && <TabsTrigger value="modules">Module</TabsTrigger>}
           </TabsList>
 
@@ -1087,6 +1089,11 @@ export default function AdminPage() {
             </TabsContent>
           )}
 
+          {isSuperAdmin && (
+            <TabsContent value="quotas">
+              <QuotaTab />
+            </TabsContent>
+          )}
           {isSuperAdmin && (
             <TabsContent value="retention">
               <RetentionTab />