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

backend/app/models/transaction.py

@@ -0,0 +1,9 @@
+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)
+