refactor(backend): solve copilot comments

backend/app/models/transaction.py

@@ -1,5 +1,5 @@
 from sqlalchemy import Column, Integer, String, Float
-from app.core.db import Base
+from app.core.base import Base
 
 class Transaction(Base):
     __tablename__ = "transaction"

backend/app/models/user.py

@@ -1,6 +1,6 @@
 from sqlalchemy import Column, String
 from fastapi_users.db import SQLAlchemyBaseUserTableUUID
-from app.core.base import Base  # Import Base z base.py
+from app.core.base import Base
 
 class User(SQLAlchemyBaseUserTableUUID, Base):
     first_name = Column(String(length=100), nullable=True)