feat: enhance database session management with commit and rollback; add user and deal API tests

app/core/database.py

@@ -14,4 +14,9 @@ AsyncSessionMaker = async_sessionmaker(bind=engine, expire_on_commit=False)
 async def get_session() -> AsyncGenerator[AsyncSession, None]:
     """Yield an async database session for request scope."""
     async with AsyncSessionMaker() as session:
-        yield session
+        try:
+            yield session
+            await session.commit()
+        except Exception:  # pragma: no cover - defensive cleanup
+            await session.rollback()
+            raise