fix(smtp): keep a copy of every submitted message in the sender's Sent folder

A message sent over SMTP submission was delivered but left nothing behind: the
account's Sent folder stayed empty. Webmail files its own copy after sending,
and a mail client cannot do it either, since IMAP APPEND is not implemented, so
nothing ever filled Sent for SMTP clients.

- ProcessOutboundEmailInput.FileSentCopy asks Submit to file the signed message
  in the sender's Sent folder, as read (not unread), sharing the blob the
  delivery jobs already reference; one copy per submission, not per recipient
- the SMTP submission session sets it; the webmail leaves it off because it
  files its own copy without the Bcc line, and asking twice would duplicate it
- filed only after the message is durably queued, so a submission refused by the
  send limit leaves nothing behind; a failure to file is logged, never returned,
  because the mail is already on its way and the client would send it again

Tests: unit cases (asked/not asked, shared blob, failure swallowed, limit
refusal, no repository) and an end-to-end test on the real submission server
that fails with "Sent holds 0 messages" without the fix. Full go test ./...
(25 packages) with TEST_DATABASE_URL, go vet and lang-lint are clean.

services/protocols/internal/presentation/smtp/submission_backend.go

@@ -122,6 +122,8 @@ func (s *submissionSession) Data(r io.Reader) error {
 		Recipients:           s.recipients,
 		Body:                 r,
 		MaxRecipientsPerHour: s.authenticated.MaxRecipientsPerHour,
+		// A client cannot file its own copy over IMAP, so the server keeps it
+		FileSentCopy: true,
 	})
 	if err == nil {
 		return nil