🗃️(backend) add missing editable=False on client_secret_sha256

Migration 0025 declared client_secret_sha256 without editable=False,
while the model field is setting it.

src/backend/core/migrations/0029_alter_application_client_secret_sha256.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.17 on 2026-10-21 20:49
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0028_room_deleted_at'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='application',
+            name='client_secret_sha256',
+            field=models.CharField(blank=True, editable=False, max_length=255, null=True),
+        ),
+    ]