refactor: restructure project into layered architecture

backend/app/db/models.py

@@ -0,0 +1,21 @@
+from sqlalchemy import Column
+from sqlalchemy import Integer
+from sqlalchemy import String
+
+from app.db.session import Base
+
+
+class Item(Base):
+
+    __tablename__ = "items"
+
+    id = Column(
+        Integer,
+        primary_key=True,
+        index=True
+    )
+
+    title = Column(
+        String,
+        nullable=False
+    )