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>

README.md

@@ -17,7 +17,8 @@ A fullstack SaaS web application built with FastAPI, React, and PostgreSQL.
 - `/api/users/me` — authenticated user info
 - `/api/profile/me` — GET/PUT personal profile (position, phone, date of birth, address)
 - Profile data stored in a dedicated `profiles` table; auto-created on first access
-- Admin role flag (`is_superuser`) stored in `users` table; hidden from all API responses
+- Admin role flag (`is_superuser`) stored in `users` table; exposed as `is_admin` in API (false for regular users, true for admins)
+- Admin-only user management at `/admin`: list all users, add users, delete users, toggle active status
 - All input sanitized before reaching the DB (null-byte rejection, length caps, format validation)
 - 3 separate Docker containers: `db` (PostgreSQL), `backend` (FastAPI), `frontend` (nginx)
 - All containers run as non-root users (UID 1001 for backend and frontend, UID 70 for db)