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/message_headers.go

@@ -19,10 +19,15 @@ type MessageHeaders struct {
 	Subject         string
 	FromDisplayName string
 	// FromAddress is the address in From, lowercased, or empty when it does not parse.
-	FromAddress string
-	MessageID       string
-	Snippet         string
-	HasAttachments  bool
+	FromAddress    string
+	MessageID      string
+	Snippet        string
+	HasAttachments bool
+	// InReplyTo and References are the Message-IDs this message answers, with
+	// the angle brackets removed so they compare equal to MessageID. They are
+	// what ties a reply to the conversation it belongs to.
+	InReplyTo  string
+	References []string
 }
 
 // wordDecoder tolerates the charsets that turn up in real mail. An unknown
@@ -57,6 +62,10 @@ func ExtractMessageHeaders(payload []byte) MessageHeaders {
 
 	out.Subject = truncateUTF8(decodeHeader(msg.Header.Get("Subject")), 998)
 	out.MessageID = truncateUTF8(strings.Trim(msg.Header.Get("Message-ID"), "<>"), 998)
+	if ids := messageIDsIn(msg.Header.Get("In-Reply-To")); len(ids) > 0 {
+		out.InReplyTo = ids[0]
+	}
+	out.References = messageIDsIn(msg.Header.Get("References"))
 
 	if from := msg.Header.Get("From"); from != "" {
 		if addr, err := mail.ParseAddress(from); err == nil {
@@ -152,3 +161,23 @@ func truncateUTF8(s string, max int) string {
 	}
 	return cut
 }
+
+// maxReferences bounds how much of a long chain is kept. The ids nearest the
+// end name the messages closest to this one, which is where its parent is.
+const maxReferences = 20
+
+// messageIDsIn lists the <id> tokens of an In-Reply-To or References header,
+// without their brackets, in the order written.
+func messageIDsIn(header string) []string {
+	var ids []string
+	for _, field := range strings.Fields(header) {
+		id := strings.Trim(field, "<>,;")
+		if id != "" && len(id) <= 998 && strings.Contains(field, "<") {
+			ids = append(ids, id)
+		}
+	}
+	if len(ids) > maxReferences {
+		ids = ids[len(ids)-maxReferences:]
+	}
+	return ids
+}