Completes the email unification: the outbound relay is now one smtp
EmailProviderConnection flagged uses_relay, so a single table drives both the
Notification Bus and the Postfix relay. The legacy single-row email_relay_config
is folded in on first boot and kept only as a read fallback.
- EmailRelayService rewritten over the unified table: get/save/disable/test all
operate on the uses_relay smtp connection; password stays Fernet-encrypted in
credentials_json; masked round-trips preserve the stored secret. Postfix apply
unchanged (no-op when Postfix absent).
- migrate_legacy_config(): idempotent fold of email_relay_config → connection
(password copied verbatim — same Fernet key); wired into the boot-time legacy
migration block.
- API: PUT /notifications/admin/email-providers/:id toggles usage flags
(uses_notifications/uses_relay/relay_priority); uses_relay honored for SMTP only.
Verified: 6 new relay tests + usage-flag + notification-bus suites pass (30 passed).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
