Add pytest fixture for database session in tests

Introduce a `db_session` pytest fixture to provide a fresh SQLite in-memory database for each test function. This ensures isolated and consistent database state across tests by setting up and tearing down the database automatically.

backend/tests/conftest.py

@@ -0,0 +1,22 @@
+# tests/conftest.py
+import pytest
+from sqlalchemy.orm import Session, sessionmaker
+
+from app.utils.test_utils import setup_test_db, teardown_test_db
+
+@pytest.fixture(scope="function")
+def db_session():
+    """
+    Creates a fresh SQLite in-memory database for each test function.
+    
+    Yields a SQLAlchemy session that can be used for testing.
+    """
+    # Set up the database
+    test_engine, TestingSessionLocal = setup_test_db()
+
+    # Create a session
+    with TestingSessionLocal() as session:
+        yield session
+
+    # Clean up
+    teardown_test_db(test_engine)