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Secrets (smtp.password, oidc.client_secret, ldap.bind_password, ansible ssh_private_key/become_password/vault_password) were stored plaintext in app_settings. Add transparent encryption-at-rest: - steward/core/crypto.py: Fernet keyed off the app secret (/data/secret.key), enc:v1: prefix marks ciphertext; passthrough for plaintext/empty/no-key, never reveals plaintext on a wrong key. - settings.py: SECRET_KEYS registry; set_setting encrypts on write; all read paths (get_setting / get_all_settings / load_settings_sync) decrypt transparently; migrate_plaintext_secrets() converts legacy rows in place. - app.py startup: init_crypto(SECRET_KEY) + one-time legacy-secret migration before settings load. - Add cryptography dependency. UI masking is unchanged (it checks decrypted truthiness). Key-loss caveat documented: secrets are unrecoverable if the app secret key is lost. Unit tests cover round-trip, empty/plaintext passthrough, and wrong-key safety. Task #580. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>