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/components/Nav.tsx

@@ -1,8 +1,11 @@
 import { Link } from "react-router-dom";
+import { useQuery } from "@tanstack/react-query";
 import { useAuth } from "../hooks/useAuth";
+import { getMe } from "../api/client";
 
 export default function Nav() {
   const { logout } = useAuth();
+  const { data: user } = useQuery({ queryKey: ["me"], queryFn: getMe });
 
   return (
     <nav style={{
@@ -15,6 +18,7 @@ export default function Nav() {
       <Link to="/">Home</Link>
       <Link to="/apps">Apps</Link>
       <Link to="/settings">Settings</Link>
+      {user?.is_admin && <Link to="/admin">Admin</Link>}
       <button
         onClick={logout}
         style={{ marginLeft: "auto", cursor: "pointer" }}