Phase 3, Task 3.1. Three cookies:
- hermes_session_at: OAuth access token (HttpOnly, TTL = token TTL)
- hermes_session_rt: OAuth refresh token (HttpOnly, 30d max-age)
- hermes_session_pkce: PKCE state + verifier + provider hint (10min)
All SameSite=Lax + Path=/. Secure flag is set ONLY when the request
scheme is https — uvicorn proxy_headers=True (enabled in gated mode at
Phase 3.5) rewrites scheme from X-Forwarded-Proto so Fly's TLS
terminator works.