feat(migration): add migration to remove invalid past marker from vocab_srs_item

backend/migrations-active/20260909000001-remove-invalid-past-marker-srs-sentence.cjs

@@ -0,0 +1,13 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface) {
+    await queryInterface.sequelize.query(`
+      DELETE FROM community.vocab_srs_item
+      WHERE lower(trim(learning)) = 'vergangenheit'
+        AND lower(trim(reference)) IN ('ni-kaon ko ganiha', 'nikaon ko ganiha');
+    `);
+  },
+
+  async down() {}
+};