refactor: improve code structure and optimize key functions

backend/app/models/user.py

@@ -0,0 +1,19 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, String, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base import Base
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    username: Mapped[str] = mapped_column(String(50), unique=True, index=True, nullable=False)
+    hashed_password: Mapped[str] = mapped_column(String(255), nullable=False)
+    role: Mapped[str] = mapped_column(String(20), nullable=False)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, server_default="true")
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())