refactor: improve code structure and optimize key functions

backend/app/db/base.py

@@ -0,0 +1,5 @@
+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    pass

backend/app/db/session.py

@@ -0,0 +1,6 @@
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from app.core.config import settings
+
+engine = create_async_engine(settings.DATABASE_URL, future=True, echo=False)
+SessionLocal = async_sessionmaker(bind=engine, class_=AsyncSession, expire_on_commit=False)