✨(backend) persist user preferences for room defaults on the User model

Add attributes on the User model to persist per-user preferences for
the default link access level and the default room configuration.

The frontend will let users update these preferences and then reuse
them when generating a link through the webapp.

Persisting them on the backend (rather than in application memory
only) ensures the preferences survive across sessions and devices.

src/backend/core/migrations/0022_user_default_room_access_level_and_more.py

@@ -0,0 +1,23 @@
+# Generated by Django 5.2.14 on 2026-08-03 13:40
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0021_recording_external_process_id_alter_recording_status'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='default_room_access_level',
+            field=models.CharField(blank=True, choices=[('public', 'Public Access'), ('trusted', 'Trusted Access'), ('restricted', 'Restricted Access')], help_text='Access level applied by default to new rooms created by this user. When empty, the instance default is used.', max_length=50, null=True, verbose_name='default room access level'),
+        ),
+        migrations.AddField(
+            model_name='user',
+            name='default_room_configuration',
+            field=models.JSONField(blank=True, default=dict, help_text='Configurations applied by default to new rooms created by this user.', verbose_name='default room configuration'),
+        ),
+    ]