Phase 0, Task 0.3. start_server now computes should_require_auth(host,
allow_public) and records it on app.state.auth_required BEFORE the
existing legacy SystemExit guard fires. This gives middleware, the SPA
token-injection path, and WS endpoints a consistent read source for
'is the gate active'. The flag is set but no one reads it yet — Phase 3
registers the gate middleware.
Note: 4 pre-existing test failures in tests/hermes_cli/test_web_server.py
(PtyWebSocket) + test_update_hangup_protection.py reproduce on pristine
HEAD and are unrelated to this change (starlette TestClient WS regression).
