AUTHENTICATIONFAILED — AUTHENTICATIONFAILED

IMAP authentication failed — the username or password (or OAuth token) was rejected. To fix this, verify the credentials. If it's a password account, prompt the user to update their password and then re-create the account in Riposte. If it's OAuth (e.g., Gmail via IMAP), the token may have expired or been revoked; run the OAuth flow again via /auth/sessions to refresh credentials. In the meantime, pause any activity on this account to avoid lockout.