Add admin user management with role-gated access

Backend:
- schemas/user.py: is_admin (validation_alias=is_superuser) on UserOut and
  UserAdminOut; UserAdminCreate extends UserCreate with is_admin flag
- deps.py: get_current_admin dependency — 403 for non-superusers
- routers/admin.py: GET/POST /api/admin/users, DELETE and PATCH /active per
  user; self-delete and self-deactivate blocked
- main.py: register /api/admin router
- scripts/seed.py: seed test user with is_superuser=True; promotes existing
  user if already created without the flag

Frontend:
- api/client.ts: UserData type with is_admin, admin API functions
- components/Nav.tsx: Admin link visible only when user.is_admin is true
- pages/AdminPage.tsx: user table with add-user form, delete, toggle active
- App.tsx: AdminRoute guard (403-redirects non-admins to /); /admin route

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

frontend/src/api/client.ts

@@ -20,7 +20,35 @@ export const register = (email: string, password: string, full_name?: string) =>
   api.post("/auth/register", { email, password, full_name }).then((r) => r.data);
 
 // --- Users ---
-export const getMe = () => api.get("/users/me").then((r) => r.data);
+export interface UserData {
+  id: string;
+  email: string;
+  full_name: string | null;
+  is_active: boolean;
+  is_admin: boolean;
+}
+
+export const getMe = () => api.get<UserData>("/users/me").then((r) => r.data);
+
+// --- Admin ---
+export interface AdminUserCreate {
+  email: string;
+  password: string;
+  full_name?: string;
+  is_admin?: boolean;
+}
+
+export const adminGetUsers = () =>
+  api.get<UserData[]>("/admin/users").then((r) => r.data);
+
+export const adminCreateUser = (data: AdminUserCreate) =>
+  api.post<UserData>("/admin/users", data).then((r) => r.data);
+
+export const adminDeleteUser = (userId: string) =>
+  api.delete(`/admin/users/${userId}`);
+
+export const adminToggleActive = (userId: string) =>
+  api.patch<UserData>(`/admin/users/${userId}/active`).then((r) => r.data);
 
 // --- Profile ---
 export interface ProfileData {