fix(imap): a folder missing from one LIST no longer deletes its mail

MailboxSyncer removed every stored folder the server's LIST did not name, and
message.mailbox_id cascades, so an empty or partial LIST, or a rename done in
another client, deleted every message of those folders in one poll.

Now an empty listing is refused outright, a listing that would leave out most
of the stored folders is refused like VanishedMessageReconciler's bulk guard,
and a folder that is really gone is only marked (mailbox.missing_since and
missing_syncs, new migration) and skipped by the sync loop. It is removed only
after three consecutive listings and 24 hours, and its messages go through
MessageEraser so JMAP clients get a destroyed change for each. A new folder
with the same special use or the same UIDVALIDITY as exactly one missing
folder is treated as a rename: the row keeps its mail and takes the new path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/Version20260923101100.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * A folder that drops out of the server's LIST is marked, not deleted.
+ *
+ * MailboxSyncer used to remove any mailbox row the listing did not name, and
+ * message.mailbox_id cascades — so one partial or empty LIST, or a rename done
+ * in another client, deleted every message of the folder. The mark and the
+ * counter let the syncer wait for the absence to last before believing it.
+ */
+final class Version20260923101100 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add mailbox.missing_since and mailbox.missing_syncs.';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE mailbox ADD missing_since TIMESTAMP(0) WITHOUT TIME ZONE DEFAULT NULL');
+        $this->addSql('ALTER TABLE mailbox ADD missing_syncs INT DEFAULT 0 NOT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE mailbox DROP missing_since');
+        $this->addSql('ALTER TABLE mailbox DROP missing_syncs');
+    }
+}