feat(protocols): conversations - thread_id and in_reply_to written at delivery, list and read APIs

A reply now joins the conversation of the message it answers (Message-ID,
In-Reply-To, References), falling back to the subject for replies with no
headers. /mail/threads lists a folder as conversations and /mail/thread/{id}
returns one. Replying from the webmail flags the answered message.

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

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

@@ -187,6 +187,8 @@ func (uc *ProcessOutboundEmailUseCase) fileSentCopy(ctx context.Context, input P
 		FromDisplayName:  headers.FromDisplayName,
 		MessageIDHeader:  headers.MessageID,
 		HasAttachments:   headers.HasAttachments,
+		InReplyTo:        headers.InReplyTo,
+		References:       headers.References,
 		// The sender's own copy is not unread mail
 		AlreadySeen: true,
 	}})