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>

migrations/0017_conversation_threading.down.sql

@@ -0,0 +1,3 @@
+DROP INDEX IF EXISTS idx_msg_mailbox_thread;
+DROP FUNCTION IF EXISTS lm_has_reply_prefix(TEXT);
+DROP FUNCTION IF EXISTS lm_subject_key(TEXT);