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/presentation/http/mail_api.go

@@ -102,6 +102,47 @@ func (m *mailAPI) handleMessages(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, messages)
 }
 
+// handleThreads lists the folder as conversations, one row each.
+func (m *mailAPI) handleThreads(w http.ResponseWriter, r *http.Request) {
+	session, ok := m.authenticate(w, r)
+	if !ok {
+		return
+	}
+	query := r.URL.Query()
+	folder := query.Get("folder")
+	if folder == "" {
+		folder = "inbox"
+	}
+	limit, _ := strconv.Atoi(query.Get("limit"))
+	offset, _ := strconv.Atoi(query.Get("offset"))
+
+	threads, err := m.useCase.ListThreads(r.Context(), session.Email, folder, query.Get("q"), limit, offset)
+	if err != nil {
+		m.fail(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, threads)
+}
+
+// handleThread returns the messages of one conversation, oldest first.
+func (m *mailAPI) handleThread(w http.ResponseWriter, r *http.Request) {
+	session, ok := m.authenticate(w, r)
+	if !ok {
+		return
+	}
+	threadID := strings.TrimPrefix(r.URL.Path, "/api/v1/mail/thread/")
+	folder := r.URL.Query().Get("folder")
+	if folder == "" {
+		folder = "inbox"
+	}
+	messages, err := m.useCase.ThreadMessages(r.Context(), session.Email, folder, threadID)
+	if err != nil {
+		m.fail(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, messages)
+}
+
 func (m *mailAPI) handleMessage(w http.ResponseWriter, r *http.Request) {
 	session, ok := m.authenticate(w, r)
 	if !ok {
@@ -482,6 +523,8 @@ func (m *mailAPI) decodeCompose(
 // fail maps domain errors onto status codes without leaking internals.
 func (m *mailAPI) fail(w http.ResponseWriter, err error) {
 	switch {
+	case errors.Is(err, usecase.ErrConversationsDisabled):
+		writeError(w, http.StatusNotImplemented, "CONVERSATIONS_DISABLED", "Conversations are not enabled")
 	case errors.Is(err, postgres.ErrMessageNotFound), errors.Is(err, usecase.ErrNoSuchMailbox):
 		writeError(w, http.StatusNotFound, "NOT_FOUND", "Message not found")
 	case errors.Is(err, usecase.ErrNoRecipients):