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/scripts/seed.py

@@ -16,18 +16,27 @@ TEST_NAME = "Test User"
 async def seed() -> None:
     async with AsyncSessionLocal() as db:
         result = await db.execute(select(User).where(User.email == TEST_EMAIL))
-        if result.scalar_one_or_none():
-            print(f"[seed] test user already exists: {TEST_EMAIL}")
+        existing = result.scalar_one_or_none()
+
+        if existing:
+            # Ensure the dev test user is always an admin
+            if not existing.is_superuser:
+                existing.is_superuser = True
+                await db.commit()
+                print(f"[seed] promoted test user to admin: {TEST_EMAIL}")
+            else:
+                print(f"[seed] test user already exists: {TEST_EMAIL}")
             return
 
         user = User(
             email=TEST_EMAIL,
             hashed_password=hash_password(TEST_PASSWORD),
             full_name=TEST_NAME,
+            is_superuser=True,
         )
         db.add(user)
         await db.commit()
-        print(f"[seed] created test user — email: {TEST_EMAIL}  pwd: {TEST_PASSWORD}")
+        print(f"[seed] created test admin — email: {TEST_EMAIL}  pwd: {TEST_PASSWORD}")
 
 
 if __name__ == "__main__":