feat(01-01): add test scaffold, health and static asset tests, fix deprecated APIs

- requirements-dev.txt with pytest and httpx
- tests/conftest.py: client and tmp_data_dir fixtures
- tests/test_health.py: GET /health returns 200 with {"status": "ok"}
- tests/test_static.py: no CDN URLs in templates, dashboard returns 200
- Fix imptune/api/pages.py: use request= kwarg in TemplateResponse (Starlette compat)
- Fix imptune/main.py: replace deprecated on_event with asynccontextmanager lifespan

tests/test_health.py

@@ -0,0 +1,5 @@
+def test_health_returns_200(client):
+    """GET /health returns 200 with {"status": "ok"}."""
+    response = client.get("/health")
+    assert response.status_code == 200
+    assert response.json() == {"status": "ok"}