test: use sqlite for unit tests

Co-Authored-By: Chris Weaver <chris@onyx.app>

backend/tests/unit/onyx/db/conftest.py

@@ -1,10 +1,18 @@
 import pytest
-from sqlalchemy.orm import Session
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session, sessionmaker
 
-from onyx.db.engine import get_session_context_manager
+from onyx.db.models import Base
 
 
 @pytest.fixture
 def db_session() -> Session:
-    with get_session_context_manager() as session:
+    engine = create_engine("sqlite:///:memory:")
+    Base.metadata.create_all(engine)
+    SessionLocal = sessionmaker(bind=engine)
+    session = SessionLocal()
+    try:
         yield session
+    finally:
+        session.close()
+        Base.metadata.drop_all(engine)