feat: scaffold FastAPI backend with ports-and-adapters layout

backend/tests/test_health.py

@@ -0,0 +1,12 @@
+"""Smoke test for the application factory."""
+
+from fastapi.testclient import TestClient
+
+from app.main import create_app
+
+
+def test_health_reports_mode() -> None:
+    client = TestClient(create_app())
+    response = client.get("/api/health")
+    assert response.status_code == 200
+    assert response.json()["mode"] in ("live", "demo")