feat: add back_design field to Project model

backend/apps/projects/migrations/0004_project_back_design.py

@@ -0,0 +1,19 @@
+# Generated by Django 5.1.4 on 2026-06-03 13:58
+
+import apps.projects.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('projects', '0003_libraryasset'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='project',
+            name='back_design',
+            field=models.JSONField(default=apps.projects.models.default_back_design),
+        ),
+    ]