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/lib/api/index.ts

@@ -46,6 +46,7 @@ export type {
   CreateTenantResponse,
   TenantLDAPConfig,
   LDAPSyncResult,
+  TenantUsageEntry,
 } from "./tenants";
 export {
   getTenants,
@@ -70,6 +71,8 @@ export {
   deleteAdminTenantLDAPConfig,
   testAdminTenantLDAPConfig,
   syncAdminTenantLDAP,
+  getAllTenantUsage,
+  setTenantQuota,
 } from "./tenants";
 
 export type {