feat: bootstrap backend MVP and architecture docs

tests/test_healthz.py

@@ -0,0 +1,10 @@
+from fastapi.testclient import TestClient
+
+from app.main import app
+
+
+def test_healthz() -> None:
+    client = TestClient(app)
+    resp = client.get("/healthz")
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "ok"