feat: add restart server action for vendor and tenant

Vendor: POST /api/vendor/tenants/{id}/restart (platform:admin scope)
Tenant: POST /api/tenant/server/restart (tenant:manage scope)

Both call TenantProvisioner.stop() then start() on the server + UI
containers. Restart button on vendor TenantDetailPage (Actions card)
and tenant TenantDashboardPage (Server card). Allowed in any status
including PROVISIONING.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/tenant-hooks.ts

@@ -10,6 +10,14 @@ export function useTenantDashboard() {
   });
 }
 
+export function useRestartServer() {
+  const qc = useQueryClient();
+  return useMutation<void, Error, void>({
+    mutationFn: () => api.post('/tenant/server/restart'),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['tenant', 'dashboard'] }),
+  });
+}
+
 export function useTenantLicense() {
   return useQuery<TenantLicenseData>({
     queryKey: ['tenant', 'license'],