fix(imap): stop re-probing genuine multi-folder copies every sync

repairRelocated() probed every row whose Message-ID was also filed in another
folder. A message that genuinely lives in two folders survived every probe,
collapse() returned 0, and it was back on the work list next sync, costing a
round trip per copy per poll and filling the 100-row slice so real ghosts
behind it waited. Confirmed copies now get message.copies_confirmed_at (new
migration) and are left out of findMessageIdsAlsoFiledElsewhere() for 30 days.

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

migrations/Version20260923101300.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Remember messages the server confirmed are genuinely in two folders.
+ *
+ * The relocated-duplicates repair probed every such copy over IMAP on every
+ * sync, found them all still there, and did the same again next time. Null on
+ * every existing row, which is what makes each one be checked once.
+ */
+final class Version20260923101300 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add message.copies_confirmed_at.';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE message ADD copies_confirmed_at TIMESTAMP(0) WITHOUT TIME ZONE DEFAULT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE message DROP copies_confirmed_at');
+    }
+}