Verify private push delivery against a real Mercure hub

Problem/Motivation

The alpha publisher has mocked transport tests, but real hub and browser behavior remains unverified.

Proposed resolution

Add an end-to-end test environment using a Mercure hub and the same-origin proxy arrangement.

Remaining tasks

Verify anonymous and authenticated subscriptions, private delivery, reconnects, credential expiry, logout and permission changes. Document a working deployment configuration.