feat(web/m7-user-mgmt): /admin/users page + AdminTabs entry

New admin route /admin/users with two sections:

- Accounts — table of all users (username, optional display name,
  admin badge, created_at). Per-row "Make admin" / "Remove admin"
  button calls PUT /api/admin/users/{id}/admin. Last-admin guard
  surfaces as a toast ("Can't remove the last admin — promote
  someone else first").

- Invites — list of active invites with Copy + Revoke per row.
  "Generate invite" button creates a 24h token. Operator shares
  the token with the invitee, who enters it on /register.

AdminTabs gains a "Users" tab (5 tabs total). New typed client
functions in admin.ts (listUsers, updateUserAdmin, listInvites,
createInvite, deleteInvite) plus query factory functions
createAdminUsersQuery / createAdminInvitesQuery and query keys
qk.adminUsers / qk.adminInvites.

Tests cover: list rendering, admin badge, promote/demote actions,
last-admin guard toast, invite generation, revoke flow, empty states.
AdminTabs.test.ts updated to assert 5 tabs and Users active state.

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

web/src/lib/api/admin.ts

@@ -377,3 +377,59 @@ export function createScanScheduleQuery() {
     staleTime: 60_000
   });
 }
+
+// Admin user-management ------------------------------------------------------
+
+export type AdminUser = {
+  id: string;
+  username: string;
+  display_name: string | null;
+  is_admin: boolean;
+  created_at: string;
+};
+
+export type AdminInvite = {
+  token: string;
+  invited_by: string;
+  note: string | null;
+  created_at: string;
+  expires_at: string;
+  redeemed_at: string | null;
+  redeemed_by: string | null;
+};
+
+export async function listUsers(): Promise<AdminUser[]> {
+  const res = await api.get<{ users: AdminUser[] }>('/api/admin/users');
+  return res.users;
+}
+
+export async function updateUserAdmin(id: string, isAdmin: boolean): Promise<AdminUser> {
+  return api.put<AdminUser>(`/api/admin/users/${id}/admin`, { is_admin: isAdmin });
+}
+
+export async function listInvites(): Promise<AdminInvite[]> {
+  const res = await api.get<{ invites: AdminInvite[] }>('/api/admin/invites');
+  return res.invites;
+}
+
+export async function createInvite(note?: string): Promise<AdminInvite> {
+  return api.post<AdminInvite>('/api/admin/invites', note ? { note } : {});
+}
+
+export async function deleteInvite(token: string): Promise<void> {
+  await api.del(`/api/admin/invites/${token}`);
+}
+
+export function createAdminUsersQuery() {
+  return createQuery({
+    queryKey: qk.adminUsers(),
+    queryFn: listUsers
+  });
+}
+
+export function createAdminInvitesQuery() {
+  return createQuery({
+    queryKey: qk.adminInvites(),
+    queryFn: listInvites
+  });
+}