test: upgrade backend test suite, implement unit tests, and modernize CRUD patterns

backend/tests/conftest.py

@@ -5,15 +5,16 @@ from sqlmodel import SQLModel
 from sqlalchemy.orm import sessionmaker
 from httpx import AsyncClient, ASGITransport
 from app.main import app
+from app.database import get_session
 
-@pytest_asyncio.fixture(scope="session")
+@pytest_asyncio.fixture(scope="function")
 async def engine():
-    """Create a single in-memory SQLite engine for the entire test session."""
+    """Create a fresh in-memory SQLite engine for each test."""
     engine = create_async_engine(
         "sqlite+aiosqlite:///:memory:",
         echo=False,
     )
-    # Create tables once for the whole session to avoid overhead
+    # Create tables for each test to ensure a clean state
     async with engine.begin() as conn:
         await conn.run_sync(SQLModel.metadata.create_all)
     yield engine
@@ -26,10 +27,10 @@ async def session_factory(engine):
 
 @pytest_asyncio.fixture
 async def db_session(session_factory):
-    """Create a database session for a test. Rolls back after each test."""
+    """Create a database session for a test."""
     async with session_factory() as session:
         yield session
-        await session.rollback()
+        # Rollback is still good practice, though engine is fresh each time
 
 @pytest.fixture
 def sample_session_data():
@@ -38,10 +39,14 @@ def sample_session_data():
         "start_time": "2024-01-15T21:30:00Z",
     }
 
-@pytest.fixture
-async def api_client(engine):
-    """Provides an AsyncClient for testing API endpoints."""
+@pytest_asyncio.fixture
+async def api_client(db_session):
+    """Provides an AsyncClient for testing API endpoints with the test database session."""
+    async def override():
+        yield db_session
+    app.dependency_overrides[get_session] = override
     async with AsyncClient(
         transport=ASGITransport(app=app), base_url="http://test"
     ) as client:
         yield client
+    app.dependency_overrides.clear()