fix: prevent app crash when frontend build is missing

StaticFiles defaults to check_dir=True, which raises at import time if
frontend/dist/assets is missing on restart — taking down /healthz and
/auth/* along with the frontend. Pass check_dir=False so the mount never
crashes the app, and make the SPA fallback return a clear 503 instead of
an unhandled 500 when index.html is absent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

backend/tests/test_frontend_serving.py

@@ -14,3 +14,13 @@ async def test_auth_routes_still_work_alongside_spa_fallback(client):
         "/auth/register", json={"username": "frontendcheck", "password": "spookyspooky"}
     )
     assert response.status_code == 201
+
+
+@pytest.mark.asyncio
+async def test_missing_frontend_build_returns_clear_error(client, monkeypatch, tmp_path):
+    import app.main as main_module
+
+    monkeypatch.setattr(main_module, "FRONTEND_DIST", tmp_path)
+    response = await client.get("/some/route")
+    assert response.status_code == 503
+    assert "npm run build" in response.json()["detail"]