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/infrastructure/postgres/inbound_message_repository.go

@@ -124,6 +124,11 @@ func persistOne(ctx context.Context, tx pgx.Tx, input port.PersistInboundMessage
 	// id and received_at come back because message_flags is keyed on both:
 	// email_messages is partitioned by received_at, so the flag rows cannot be
 	// written without the same timestamp the row landed on.
+	threadID, err := resolveThread(ctx, tx, input)
+	if err != nil {
+		return 0, err
+	}
+
 	var messageID uuid.UUID
 	var receivedAt time.Time
 	err = tx.QueryRow(ctx, `
@@ -131,8 +136,8 @@ func persistOne(ctx context.Context, tx pgx.Tx, input port.PersistInboundMessage
 			mailbox_id, folder_id, uid, modseq, blob_id,
 			sender_address, recipient_addresses, size_bytes,
 			subject, snippet, from_display_name, message_id_header, has_attachments,
-			spf_result, dkim_result, dmarc_result
-		) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16)
+			spf_result, dkim_result, dmarc_result, in_reply_to, thread_id
+		) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18)
 		RETURNING id, received_at
 	`, input.MailboxID, folderID, uid, modseq, input.Blob.ID,
 		input.SenderAddress, []string{input.RecipientAddress}, input.Blob.SizeBytes,
@@ -147,7 +152,8 @@ func persistOne(ctx context.Context, tx pgx.Tx, input port.PersistInboundMessage
 		// Sent copy and drafts have no authentication results by definition -
 		// and it failed the whole insert. Sent stayed empty because that write
 		// is deliberately non-fatal, and saving a draft reported an error.
-		nullIfEmpty(input.SPFResult), nullIfEmpty(input.DKIMResult), nullIfEmpty(input.DMARCResult)).
+		nullIfEmpty(input.SPFResult), nullIfEmpty(input.DKIMResult), nullIfEmpty(input.DMARCResult),
+		nullIfEmpty(input.InReplyTo), threadID).
 		Scan(&messageID, &receivedAt)
 	if err != nil {
 		return 0, fmt.Errorf("insert email_messages: %w", err)