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/mail_store.go

@@ -426,7 +426,7 @@ func (s *MailStore) copyOne(ctx context.Context, sourceFolderID string, sourceUI
 		SELECT $2::uuid, $3::uuid, $4::bigint, $5::bigint, blob_id,
 			sender_address, recipient_addresses, size_bytes,
 			spf_result, dkim_result, dmarc_result,
-			from_display_name, subject, snippet, message_id_header, in_reply_to, thread_id, has_attachments,
+			from_display_name, subject, snippet, message_id_header, in_reply_to, COALESCE(thread_id, id), has_attachments,
 			arc_result, dane_status, tls_version, spam_score, spam_verdict, virus_name
 		  FROM email_messages WHERE id = $1 AND received_at = $6
 		RETURNING id, received_at`,