feat(email): every email is a row in email_outbox until the server accepts it (#5506)

#5506 "Email outbox: a table of outgoing email and a sender that retries
until accepted, for up to an hour" (milestone 501).

- email_outbox (migration 0042) holds each message per recipient, its
  words, its state (held / pending / sending / sent / failed), attempts and
  the server's last answer. email_outbox_events links alert events for
  #5507.
- core/outbox.py: enqueue; a sender task every 15 s claims what is due
  (SKIP LOCKED, stale claims released), sends the batch over one
  connection with no session held, and settles each row:
  - accepted -> sent;
  - a 5xx refusal -> failed at once with the server's words;
  - no connection, a timeout, a 4xx -> retried after 30 s, 1, 2, 5, 10,
    15 min, the last try at the hour, then failed;
  - a test (retry=False) settles on its first answer.
  It runs on startup, so a row left by a restart goes out at boot.
- notifications.smtp_open is the one place a connection is opened.
- mail.message takes the html part as optional.
- cleanup ages email_outbox rows on the data window.

Senders move onto it in #5507 and #5508.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/models/__init__.py

@@ -12,6 +12,7 @@ from .ansible_inventory import AnsibleTarget, AnsibleGroup, ansible_target_group
 from .ansible_schedule import AnsibleSchedule
 from .settings import AppSetting
 from .dashboard import Dashboard, DashboardWidget, DashboardShareToken
+from .outbox import EmailOutbox, email_outbox_events
 
 __all__ = [
     "Base", "User", "ApiToken", "UserInvite", "PasswordReset",
@@ -24,4 +25,5 @@ __all__ = [
     "AnsibleSchedule",
     "AppSetting",
     "Dashboard", "DashboardWidget", "DashboardShareToken",
+    "EmailOutbox", "email_outbox_events",
 ]