refactor(mail): one send path for Sent, a fixed lock order, and concurrency tests

- the webmail's Send no longer files its own Sent copy: it asks Submit to
  (FileSentCopy), the same path SMTP clients use, so the rule exists once
- every operation now locks the folders first (lowest id first) and the message
  after. Opposite moves, and a move racing a flag change, deadlocked (SQLSTATE
  40P01) because they took those locks in different orders; the new test
  reproduces it (10-19 s of retries, errors) and passes in ~1 s with the order
- TestOneMailboxEverywhere drives SMTP, IMAP and the webmail on one mailbox,
  mixing them, and checks after each step that every view agrees with the others
  and with the rows. It fails on the previous code and passes now.

services/protocols/internal/application/usecase/webmail_compose_test.go

@@ -197,6 +197,8 @@ func newSendableUseCaseWith(
 	}
 	submission := NewProcessOutboundEmailUseCase(
 		nil, &countingOutboundRepo{}, &capturingBlobStorage{}, &recordingSigner{})
+	// The Sent copy is filed by the submission, as it is for SMTP clients
+	submission.SetSentCopies(messages)
 	return (&WebmailUseCase{
 		repo:       repo,
 		auth:       &stubAuthRepo{account: account},