chore: relocate backend into apps/api ahead of monorepo merge

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/api/utils/courses/junction.util.js

@@ -0,0 +1,16 @@
+"use strict";
+
+/**
+ * Replace all junction rows for a course in one shot (delete + bulk create).
+ */
+async function syncJunction(Model, courseId, ids, fkField, transaction) {
+  await Model.destroy({ where: { course_id: courseId }, transaction });
+  if (ids?.length) {
+    await Model.bulkCreate(
+      ids.map((id) => ({ course_id: courseId, [fkField]: id })),
+      { transaction }
+    );
+  }
+}
+
+module.exports = { syncJunction };