Phase 3, Task 3.5. Three changes to web_server.py:
1. start_server replaces the legacy SystemExit-refusing-to-bind guard
with: if app.state.auth_required and no providers registered, exit
with a clear message; otherwise log the gate-on banner. --insecure
keeps its existing behaviour.
2. uvicorn proxy_headers flag is computed from app.state.auth_required.
Loopback / --insecure keep it False (so _ws_client_is_allowed sees
the real peer for the loopback gate); gated mode flips it True so
X-Forwarded-Proto from Fly's TLS terminator is honoured for cookie
Secure-flag decisions in detect_https().
3. _serve_index no longer injects window.__HERMES_SESSION_TOKEN__ when
the gate is on — the SPA reads identity from /api/auth/me using
cookie auth instead. window.__HERMES_AUTH_REQUIRED__ flag lets the
SPA pick between ticket-auth (gated) and token-auth (loopback) for
/api/pty + /api/ws (Phase 5 will wire this in the React layer).
4 new behavioural tests; loopback regression harness still green.
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