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);

migrations/0017_conversation_threading.up.sql

@@ -0,0 +1,41 @@
+-- Conversations.
+--
+-- email_messages.thread_id has existed since the first migration and nothing
+-- filled it, and in_reply_to was only ever copied, never written. A reply and
+-- the message it answered were therefore unrelated rows, which is why the
+-- webmail could neither show a conversation nor say whether it had been
+-- answered.
+--
+-- The subject key lives in the database so delivery, the backfill below and
+-- anything later share one definition of "the same subject": the prefixes mail
+-- clients add in several languages are not part of it.
+CREATE FUNCTION lm_subject_key(subject TEXT) RETURNS TEXT AS $$
+    SELECT btrim(regexp_replace(
+        lower(coalesce(subject, '')),
+        '^(\s*(re|fw|fwd|enc|res|rv|aw|wg|tr|antw)(\[\d+\])?\s*:)+', ''))
+$$ LANGUAGE SQL IMMUTABLE;
+
+CREATE FUNCTION lm_has_reply_prefix(subject TEXT) RETURNS BOOLEAN AS $$
+    SELECT lower(coalesce(subject, '')) ~ '^\s*(re|fw|fwd|enc|res|rv|aw|wg|tr|antw)(\[\d+\])?\s*:'
+$$ LANGUAGE SQL IMMUTABLE;
+
+-- Mail that arrived before replies carried their headers cannot be linked by
+-- Message-ID, so it is grouped by subject, and only where a reply is actually
+-- in the mailbox: two unrelated messages that merely share a subject stay apart.
+-- Everything else keeps thread_id NULL, which the webmail reads as "a
+-- conversation of its own" (COALESCE(thread_id, id)).
+WITH groups AS (
+    SELECT DISTINCT mailbox_id, lm_subject_key(subject) AS k
+      FROM email_messages
+     WHERE lm_has_reply_prefix(subject) AND lm_subject_key(subject) <> ''
+)
+UPDATE email_messages m
+   SET thread_id = md5(m.mailbox_id::text || '|' || g.k)::uuid
+  FROM groups g
+ WHERE m.mailbox_id = g.mailbox_id
+   AND lm_subject_key(m.subject) = g.k
+   AND m.thread_id IS NULL;
+
+-- The listing groups by this and filters by mailbox and folder.
+CREATE INDEX idx_msg_mailbox_thread ON email_messages (mailbox_id, (COALESCE(thread_id, id)))
+    WHERE expunged_at IS NULL;