A campaign mail is logged and attached to its campaign on MessageSending,
before the transport is called, and the attach added to the sent counter.
Rejected or suppressed recipients, provider errors and every retry of a
failed send therefore all counted as sent, putting the campaign overview far
above what the provider actually sent.

- laravel-mails: dispatch a MailSent event once a logged mail gets its
  sent_at.
- The sent counter now follows the pivot's mail_sent flag, set by the new
  CountSentCampaignMails listener on MailSent, in both the incremental
  updates and the full recalculation.
- Migration 14 marks the mails that were already sent and rebuilds the
  counters; the rebuild moves from the command into CampaignStats::rebuildAll().
- The campaign table's delivered and bounce percentages are now relative to
  the sent count instead of the mailing list's current active subscribers.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>