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>

backend/app/deps.py

@@ -30,3 +30,14 @@ async def get_current_user(
     if not user or not user.is_active:
         raise credentials_exception
     return user
+
+
+async def get_current_admin(
+    current_user: User = Depends(get_current_user),
+) -> User:
+    if not current_user.is_superuser:
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail="Admin access required",
+        )
+    return current_user