Add a duress password (DISTRESS_PASSWORD env): entering it at login trips an
in-memory full lockdown — all new logins (password + passkey) and every existing
session are refused until the server is restarted, and the login itself returns a
normal "invalid credentials" so it gives nothing away. Also add
POST /api/auth/blacklist-token as a clearly-named alias for revoking the current
JWT (same effect as signout).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>