07f2c90055
Two pre-existing bugs surfaced while reviewing #806 (kept separate per scope policy — no OIDC code here): - backup_s3_secret_key and backup_rsync_ssh_key (an SSH PRIVATE KEY) were returned in PLAINTEXT by GET /admin/backup/config and by the generic settings reads (GET /admin/settings and /admin/settings/:type — which mask the recaptcha/umami/rybbit keys but not these). All three now mask with the established bullet sentinel, and PUT /admin/backup/config skips the sentinel on write so the edit form round-trips without clobbering stored credentials (same pattern as the email/WhatsApp config endpoints) - /api/auth/admin/login/mfa was missing from the maintenance-mode allowlist: the first login step passed, the second factor got a 503 — any MFA-enrolled admin was locked out exactly while maintenance mode was on Regression tests: masking on all three read paths, sentinel round-trip preserves stored values, real rotation still writes.