fix: Mandantenverwaltung LDAP-Status und Nutzer-Listing

- tenantstore.List(): LEFT JOIN tenant_ldap hinzugefügt — ldap_enabled + ldap_url
  werden jetzt im GET /api/tenants Response mitgeliefert
- Tenant-Struct: Felder LDAPEnabled *bool + LDAPURL string ergänzt
- Neuer Endpunkt GET /api/tenants/{id}/users → listet Nutzer eines Mandanten
- api.ts: getTenantUsers() Funktion + tenant_id Feld im User Interface
- Admin-Panel: "Nutzer"-Button im Mandanten-Tab öffnet Dialog mit Nutzerliste

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

src/app/admin/page.tsx

@@ -24,6 +24,7 @@ import {
   deleteLDAPConfig,
   testLDAPConfig,
   getTenants,
+  getTenantUsers,
   createTenant,
   updateTenant,
   deleteTenant,
@@ -238,6 +239,12 @@ export default function AdminPage() {
   // Superadmin: tenant LDAP dialog
   const [tenantLdapDialogId, setTenantLdapDialogId] = useState<number | null>(null);
 
+  // Tenant users dialog
+  const [tenantUsersDialogId, setTenantUsersDialogId] = useState<number | null>(null);
+  const [tenantUsersDialogName, setTenantUsersDialogName] = useState("");
+  const [tenantUsers, setTenantUsers] = useState<User[]>([]);
+  const [tenantUsersLoading, setTenantUsersLoading] = useState(false);
+
   // Labels state
   const [adminLabels, setAdminLabels] = useState<MailLabel[]>([]);
   const [adminLabelsLoading, setAdminLabelsLoading] = useState(false);
@@ -747,6 +754,18 @@ export default function AdminPage() {
     finally { setDomainsLoading(false); }
   }
 
+  async function openUsersDialog(t: Tenant) {
+    setTenantUsersDialogId(t.id);
+    setTenantUsersDialogName(t.name);
+    setTenantUsersLoading(true);
+    setTenantUsers([]);
+    try {
+      const users = await getTenantUsers(t.id);
+      setTenantUsers(users || []);
+    } catch { /* ignore */ }
+    finally { setTenantUsersLoading(false); }
+  }
+
   async function handleAddDomain() {
     if (!domainDialogTenant || !newDomain) return;
     setAddDomainLoading(true);
@@ -2477,6 +2496,9 @@ export default function AdminPage() {
                             <Button size="sm" variant="outline" onClick={() => openDomainDialog(t)}>
                               Domains
                             </Button>
+                            <Button size="sm" variant="outline" onClick={() => openUsersDialog(t)}>
+                              Nutzer
+                            </Button>
                             <Button size="sm" variant="outline" onClick={() => setTenantLdapDialogId(t.id)}>
                               LDAP
                             </Button>
@@ -2564,6 +2586,46 @@ export default function AdminPage() {
               </DialogContent>
             </Dialog>
 
+            {/* Tenant users dialog */}
+            <Dialog open={tenantUsersDialogId !== null} onOpenChange={(open) => { if (!open) setTenantUsersDialogId(null); }}>
+              <DialogContent>
+                <DialogHeader>
+                  <DialogTitle>Nutzer: {tenantUsersDialogName}</DialogTitle>
+                  <DialogDescription>Dem Mandanten zugewiesene Benutzerkonten.</DialogDescription>
+                </DialogHeader>
+                {tenantUsersLoading ? (
+                  <Skeleton className="h-24 w-full" />
+                ) : tenantUsers.length === 0 ? (
+                  <p className="text-sm text-muted-foreground py-4 text-center">Keine Benutzer diesem Mandanten zugewiesen.</p>
+                ) : (
+                  <Table>
+                    <TableHeader>
+                      <TableRow>
+                        <TableHead>Benutzername</TableHead>
+                        <TableHead>E-Mail</TableHead>
+                        <TableHead>Rolle</TableHead>
+                        <TableHead>Status</TableHead>
+                      </TableRow>
+                    </TableHeader>
+                    <TableBody>
+                      {tenantUsers.map((u) => (
+                        <TableRow key={u.id}>
+                          <TableCell className="font-medium">{u.username}</TableCell>
+                          <TableCell className="text-sm text-muted-foreground">{u.email}</TableCell>
+                          <TableCell><Badge variant="outline">{u.role}</Badge></TableCell>
+                          <TableCell>
+                            <Badge variant={u.active ? "default" : "secondary"}>
+                              {u.active ? "Aktiv" : "Inaktiv"}
+                            </Badge>
+                          </TableCell>
+                        </TableRow>
+                      ))}
+                    </TableBody>
+                  </Table>
+                )}
+              </DialogContent>
+            </Dialog>
+
             {/* Tenant LDAP dialog (superadmin) */}
             {tenantLdapDialogId !== null && (
               <TenantLDAPDialog