The smtp_pass setting was stored as plaintext in app_settings, exposing
SMTP credentials to anyone with database read access. Apply the same
encrypt_api_key/decrypt_api_key pattern already used for OIDC client
secrets and API keys. A new migration transparently re-encrypts any
existing plaintext value on startup; decrypt_api_key handles legacy
plaintext gracefully so in-flight reads remain safe during upgrade.
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