Add test fixtures with in-memory SQLite

[ddvlamin]

Description

Create pytest fixtures for async database testing with in-memory SQLite.

Files

• Create: tests/conftest.py

Fixtures

• db_session — creates in-memory SQLite, creates all tables, yields session, disposes engine
• client — FastAPI test client with overridden get_db dependency, uses httpx AsyncClient
• sample_list — pre-created ShoppingList in the test DB

Details

• Use TEST_DATABASE_URL = "sqlite+aiosqlite://" for in-memory
• Clean up engine after each test via engine.dispose()

Acceptance Criteria

• Fixtures load without errors
• Test client can make requests and they reach the app