refactor: improve code structure and optimize key functions

backend/app/crud/user.py

@@ -0,0 +1,30 @@
+from typing import Sequence
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.user import User
+from app.schemas.user import UserCreate
+
+
+async def get_by_username(db: AsyncSession, username: str) -> User | None:
+    result = await db.execute(select(User).where(User.username == username))
+    return result.scalar_one_or_none()
+
+
+async def create(db: AsyncSession, user_in: UserCreate, hashed_password: str) -> User:
+    user = User(
+        username=user_in.username,
+        hashed_password=hashed_password,
+        role=user_in.role,
+        is_active=user_in.is_active,
+    )
+    db.add(user)
+    await db.commit()
+    await db.refresh(user)
+    return user
+
+
+async def list_users(db: AsyncSession, limit: int = 100) -> Sequence[User]:
+    result = await db.execute(select(User).limit(limit))
+    return result.scalars().all()