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_inbound_email.go

@@ -347,6 +347,8 @@ func (uc *ProcessInboundEmailUseCase) Handle(ctx context.Context, input ProcessI
 			FromDisplayName:  headers.FromDisplayName,
 			MessageIDHeader:  headers.MessageID,
 			HasAttachments:   headers.HasAttachments,
+			InReplyTo:        headers.InReplyTo,
+			References:       headers.References,
 			AlreadySeen:      alreadySeen,
 			SPFResult:        authResult.SPF,
 			DKIMResult:       authResult.DKIM,