v0.18.0 Slice 4: outbound_emails audit table + admin endpoint

Per the v0.18.0 email + webhook hygiene proposal §3:

  * `backend/migrations/020_outbound_emails.sql` — new table
    capturing every send attempt: to_address, from_address,
    subject, kind, sent_at, status ('sent' | 'failed' | 'deferred'
    | 'bounced'), error, notification_id (FK), message_id (for
    Slice 5 bounce correlation).
  * `email.record_outbound()` — best-effort write helper every
    send path calls. Status='sent' on SMTP success, 'failed' on
    exception (with class + message in `error`), 'deferred' on
    the dev-fallback path where SMTP_HOST is unset (the send
    didn't happen but the row records the attempt).
  * `email._deliver` (watcher notifications + bundles), `digest.py`,
    `email_otc.py`, `email_invite.py` — every send path now records.
  * `GET /api/admin/outbound-emails` — admin-only listing,
    filterable by kind / status / to_address. Answers "did this
    person ever get their invite?" without grepping VM logs. No
    admin UI in v0.18.0; operator queries via curl + jq for now.

7 new integration tests covering: OTC / invite / notification all
write rows with matching Message-ID; admin endpoint lists,
filters by kind, filters by to_address (case-insensitive),
refuses non-admins.

Full suite: 291 passed.

backend/app/digest.py

@@ -191,6 +191,7 @@ def assemble_for_user(
         cfg, email, subject, body,
         unsubscribe_mailto=cfg.unsubscribe_mailto,
         unsubscribe_url=unsubscribe_url,
+        kind="digest",
     )
     if not sent:
         return False