fix(imap): index unread rows per folder for the post-sync recount

Every folder sync ends with countUnseenForMailbox(), `mailbox_id = ? AND
seen_at IS NULL`, which visited every row of the folder to test seen_at. A
partial index on message(mailbox_id) WHERE seen_at IS NULL holds only the
unread rows. Migration-only, like the table's other partial indexes.

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

migrations/Version20260923101400.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * The unread count every folder sync ends with, without scanning the folder.
+ *
+ * MessageSyncer recounts `mailbox_id = ? AND seen_at IS NULL` after every
+ * sync of every folder. The unique (mailbox_id, imap_uid) index finds the
+ * folder's rows but then has to visit each one to test seen_at; a partial
+ * index holds only the unread rows, which in a well-kept mailbox is almost
+ * none of them. The total count already has an index to use.
+ *
+ * Migration-only, like the other partial indexes on this table: the ORM
+ * mapping cannot express the WHERE clause.
+ */
+final class Version20260923101400 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Partial index on message(mailbox_id) for unread rows.';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('CREATE INDEX idx_message_unseen_mailbox ON message (mailbox_id) WHERE seen_at IS NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP INDEX idx_message_unseen_mailbox');
+    }
+}