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/webmail_mail.go

@@ -37,6 +37,9 @@ type WebmailUseCase struct {
 	// the submission use case, not here.)
 	blobStore port.BlobStorage
 	messages  port.InboundMessageRepository
+	// conversations serves the conversation views and records which message a
+	// reply answered. Optional; see SetConversations.
+	conversations port.ConversationReader
 }
 
 func NewWebmailUseCase(
@@ -91,6 +94,42 @@ func (uc *WebmailUseCase) ListMessages(
 	return uc.repo.ListMessages(ctx, id, folder, search, limit, offset)
 }
 
+// SetConversations enables the conversation views. Optional: without it the
+// thread routes answer "not configured" and the webmail falls back to the
+// plain message list.
+func (uc *WebmailUseCase) SetConversations(c port.ConversationReader) {
+	uc.conversations = c
+}
+
+// ErrConversationsDisabled is returned when no conversation reader was wired.
+var ErrConversationsDisabled = errors.New("conversations are not enabled")
+
+func (uc *WebmailUseCase) ListThreads(
+	ctx context.Context, address, folder, search string, limit, offset int,
+) ([]port.WebmailThread, error) {
+	if uc.conversations == nil {
+		return nil, ErrConversationsDisabled
+	}
+	id, err := uc.mailboxID(ctx, address)
+	if err != nil {
+		return nil, err
+	}
+	return uc.conversations.ListThreads(ctx, id, folder, search, limit, offset)
+}
+
+func (uc *WebmailUseCase) ThreadMessages(
+	ctx context.Context, address, folder, threadID string,
+) ([]port.WebmailThreadMessage, error) {
+	if uc.conversations == nil {
+		return nil, ErrConversationsDisabled
+	}
+	id, err := uc.mailboxID(ctx, address)
+	if err != nil {
+		return nil, err
+	}
+	return uc.conversations.ThreadMessages(ctx, id, folder, threadID)
+}
+
 // GetMessage returns the raw RFC 5322 bytes and marks the message read.
 func (uc *WebmailUseCase) GetMessage(ctx context.Context, address, folder string, uid uint32) ([]byte, error) {
 	id, err := uc.mailboxID(ctx, address)
@@ -207,6 +246,16 @@ func (uc *WebmailUseCase) Send(ctx context.Context, input ComposeInput) error {
 	// thing. Swallowed like the Sent copy above and for the same reason: the
 	// mail is already queued, and failing the request now would only invite a
 	// second copy to be sent.
+	// The message this one answers is marked answered. Swallowed for the same
+	// reason: the reply is already on its way, and a failed flag only means the
+	// list shows the conversation as unanswered until the next message.
+	if uc.conversations != nil && cleanMessageID(input.InReplyTo) != "" {
+		if mailboxID, idErr := uc.mailboxID(ctx, account.EmailAddress); idErr == nil {
+			if err := uc.conversations.MarkAnsweredByMessageID(ctx, mailboxID, input.InReplyTo); err != nil {
+				log.Printf("webmail: could not mark %s answered: %v", input.InReplyTo, err)
+			}
+		}
+	}
 	if input.DraftUID != 0 {
 		if mailboxID, idErr := uc.mailboxID(ctx, account.EmailAddress); idErr == nil {
 			if err := uc.repo.Expunge(ctx, mailboxID, "Drafts", input.DraftUID); err != nil {
@@ -341,6 +390,8 @@ func (uc *WebmailUseCase) SaveDraft(ctx context.Context, input ComposeInput, rep
 		FromDisplayName:  headers.FromDisplayName,
 		MessageIDHeader:  headers.MessageID,
 		HasAttachments:   headers.HasAttachments,
+		InReplyTo:        headers.InReplyTo,
+		References:       headers.References,
 		AlreadySeen:      true,
 	}})
 	if err != nil {