fix: derive contingent change signal from the sync diff

migrations/Version20260911170000.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260911170000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Drops contingent_sync_state.content_hash: the rolling sync window makes a snapshot digest unusable as a change signal.';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE contingent_sync_state DROP content_hash');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE contingent_sync_state ADD content_hash VARCHAR(64) NOT NULL');
+    }
+}