Keep the integration tests green through the port

The backend integration tests are the proof that the Hono implementation behaves like the Express one, so they need to run against the new app during the port, not after it.

• Minimal harness change only: supertest can target the Hono node server (or a temporary adapter), the full Jest-to-Vitest move is a separate epic
• Every existing test passes against the ported routes before cutover
• Auth flow tests need care: cookie setting/clearing behaviour should match exactly (name, httpOnly, sameSite, maxAge)