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_test.go

@@ -1,6 +1,10 @@
 package usecase
 
-import "testing"
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
 
 func TestExtractMessageHeaders_ReadsSubjectSenderAndSnippet(t *testing.T) {
 	msg := "From: Alice Example <alice@remote.test>\r\n" +
@@ -142,3 +146,37 @@ func TestExtractMessageHeaders_AttachmentDetection(t *testing.T) {
 		})
 	}
 }
+
+func TestHeadersCarryWhatAMessageAnswers(t *testing.T) {
+	raw := "Message-ID: <c@x.test>\r\nIn-Reply-To: <b@x.test>\r\nReferences: <a@x.test>\r\n <b@x.test>\r\nSubject: Re: Plan\r\n\r\nbody"
+	h := ExtractMessageHeaders([]byte(raw))
+	if h.InReplyTo != "b@x.test" {
+		t.Errorf("InReplyTo = %q, want b@x.test", h.InReplyTo)
+	}
+	if len(h.References) != 2 || h.References[0] != "a@x.test" || h.References[1] != "b@x.test" {
+		t.Errorf("References = %v, want [a@x.test b@x.test]", h.References)
+	}
+	if h.MessageID != "c@x.test" {
+		t.Errorf("MessageID = %q", h.MessageID)
+	}
+}
+
+func TestAMessageThatAnswersNothingHasNoReferences(t *testing.T) {
+	h := ExtractMessageHeaders([]byte("Subject: Hi\r\n\r\nbody"))
+	if h.InReplyTo != "" || len(h.References) != 0 {
+		t.Errorf("a first message claims to answer %q %v", h.InReplyTo, h.References)
+	}
+}
+
+// A chain can run to hundreds of ids. The ones nearest the end name the parent.
+func TestAVeryLongChainKeepsTheNewestIDs(t *testing.T) {
+	var b strings.Builder
+	b.WriteString("References:")
+	for i := 0; i < 50; i++ {
+		fmt.Fprintf(&b, " <m%d@x.test>", i)
+	}
+	h := ExtractMessageHeaders([]byte(b.String() + "\r\nSubject: x\r\n\r\nbody"))
+	if len(h.References) != maxReferences || h.References[len(h.References)-1] != "m49@x.test" {
+		t.Errorf("kept %d ids ending %q", len(h.References), h.References[len(h.References)-1])
+	}
+}