Foundation for folding the single-row email_relay_config into the multi-row
EmailProviderConnection so one table drives notifications and the Postfix relay.
- EmailProviderConnection gains uses_notifications (default true), uses_relay
(default false), relay_priority (default 0); surfaced in to_dict.
- Notification selection (default_provider) now restricted to uses_notifications
rows, so relay-only connections are never used to send notifications.
- New relay_provider() selects the active SMTP connection by relay_priority.
- add_provider() accepts the flags; uses_relay honored only for provider=smtp
(API providers can't be a Postfix smarthost).
- Migration 038 (idempotent additive columns).
Additive — existing providers default to uses_notifications=true / no relay.
Verified: 4 new tests + notification-bus suite pass (24 passed).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
