Set up backend test infrastructure

Add pytest fixtures (engine, db_session, client) with session-scoped
event loop to avoid asyncpg loop mismatch errors. Smoke tests verify
all three main API endpoints return empty results on a clean DB.
Test DB provided by docker-compose.test.yml on port 5433.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/pyproject.toml

@@ -66,4 +66,6 @@ root = "src"
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
+asyncio_default_fixture_loop_scope = "session"
+asyncio_default_test_loop_scope = "session"
 testpaths = ["tests"]