feat(email): the report, invites, resets and test sends go through the outbox; only it talks SMTP (#5508)

#5508 "Every other email through the outbox: weekly report, invites,
password resets, and the test buttons" (milestone 501).

- outbox.send_each queues one email per reader (claimed, so the sender
  doesn't take it too) and tries them at once, for a page that answers
  with what happened; outbox.said words the result. attempt takes a cfg
  for a test of the form's unsaved settings and returns the settled rows.
- Weekly report (scheduled and Send now) and invites: queued, tried at
  once; one the server couldn't take yet is retried for an hour. An invite
  that didn't go shows its link with the reason, as before.
- Password reset: queued in the reset link's own transaction; the sender
  delivers it on its next tick.
- Settings -> Notifications test email and a rule's Send test: queued and
  recorded with retry=False, tried once (System 156 ruling).
- Removed mail.send, notifications._send_email and _smtp_send_sync;
  dispatch_notifications is webhook only. A guard asserts only the outbox
  (and smtp_open itself) opens an SMTP connection, with a self-test.

Also fixes run 8941 (#5507's integration test): its Also-send-to
addresses were "@test", which is_real_address rightly drops; they are
example.com now.

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

steward/core/mail.py

@@ -163,13 +163,6 @@ async def audience(db) -> dict:
     return out
 
 
-async def send(cfg: dict, messages: list[EmailMessage]) -> None:
-    """Send every message over one SMTP connection, off the event loop."""
-    import asyncio
-    from steward.core.notifications import _smtp_send_sync
-    await asyncio.get_running_loop().run_in_executor(None, _smtp_send_sync, cfg, messages)
-
-
 def _render(name: str, **ctx) -> tuple[str, str]:
     ctx = {"c": palette(), **ctx}
     return (_env.get_template(f"{name}.txt").render(ctx),