refactor(backend): refactor project, add database migrations support

backend/app/models/.keep

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+# SQLAlchemy modely
+

backend/app/models/transaction.py

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+from sqlalchemy import Column, Integer, String, Float
+from ..core.db import Base
+
+class Transaction(Base):
+    __tablename__ = "transaction"
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    amount = Column(Float, nullable=False)
+    description = Column(String(length=255), nullable=True)
+

backend/app/models/user.py

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