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/port/conversations.go

@@ -0,0 +1,68 @@
+package port
+
+import (
+	"context"
+	"time"
+)
+
+// WebmailParticipant is someone who wrote in a conversation.
+type WebmailParticipant struct {
+	Name    string `json:"name"`
+	Address string `json:"address"`
+	// Me marks the mailbox owner, so the list can say "me" instead of a name
+	// the reader knows perfectly well.
+	Me bool `json:"me"`
+}
+
+// WebmailThread is one row of the conversation list: the newest message in the
+// folder being looked at, with what the rest of the conversation adds.
+type WebmailThread struct {
+	ThreadID       string               `json:"thread_id"`
+	UID            uint32               `json:"uid"`
+	Subject        string               `json:"subject"`
+	SenderAddress  string               `json:"sender_address"`
+	FromName       string               `json:"from_display_name"`
+	Snippet        string               `json:"snippet"`
+	ReceivedAt     time.Time            `json:"received_at"`
+	HasAttachments bool                 `json:"has_attachments"`
+	MessageCount   int                  `json:"message_count"`
+	UnseenCount    int                  `json:"unseen_count"`
+	Seen           bool                 `json:"seen"`
+	Flagged        bool                 `json:"flagged"`
+	Answered       bool                 `json:"answered"`
+	Participants   []WebmailParticipant `json:"participants"`
+}
+
+// WebmailThreadMessage is one message inside a conversation.
+type WebmailThreadMessage struct {
+	UID            uint32    `json:"uid"`
+	Folder         string    `json:"folder"`
+	Subject        string    `json:"subject"`
+	SenderAddress  string    `json:"sender_address"`
+	FromName       string    `json:"from_display_name"`
+	Snippet        string    `json:"snippet"`
+	ReceivedAt     time.Time `json:"received_at"`
+	HasAttachments bool      `json:"has_attachments"`
+	Seen           bool      `json:"seen"`
+	Flagged        bool      `json:"flagged"`
+	// Answered is true when the mailbox owner replied, whether the \Answered
+	// flag was set or the reply is simply there in the conversation - a reply
+	// written in another mail program leaves no flag, only the message.
+	Answered bool `json:"answered"`
+	// Own marks a message the mailbox owner wrote.
+	Own bool `json:"own"`
+}
+
+// ConversationReader serves the conversation views.
+type ConversationReader interface {
+	// ListThreads returns the conversations that have a message in the folder,
+	// newest first by that message.
+	ListThreads(ctx context.Context, mailboxID, folderName, search string, limit, offset int) ([]WebmailThread, error)
+	// ThreadMessages returns every message of one conversation, oldest first.
+	// folderName is the folder being viewed: a conversation's messages in Trash
+	// and Junk are left out unless that is the folder.
+	ThreadMessages(ctx context.Context, mailboxID, folderName, threadID string) ([]WebmailThreadMessage, error)
+	// MarkAnsweredByMessageID sets \Answered on the message a reply was written
+	// to, wherever in the mailbox it is filed.
+	MarkAnsweredByMessageID(ctx context.Context, mailboxID, messageID string) error
+}

services/protocols/internal/application/port/inbound_message_repository.go

@@ -22,6 +22,10 @@ type PersistInboundMessageInput struct {
 	FromDisplayName string
 	MessageIDHeader string
 	HasAttachments  bool
+	// InReplyTo and References place the message in its conversation. Both
+	// hold Message-IDs without angle brackets.
+	InReplyTo  string
+	References []string
 	// AlreadySeen marks a copy the user themselves composed - their own Sent
 	// copy, or a draft. It is not unread mail, and counting it as such is why
 	// the Sent folder carried an unread badge that nothing could clear: no