Fix ImproperlyConfigured for scrobble-detail by backfilling NULL UUIDs

The ScrobbleSerializer uses HyperlinkedModelSerializer with
lookup_field='uuid', but 3,273 of 6,155 Scrobbles had NULL UUIDs,
causing URL resolution to fail.

Fix:
- Add migration 0104 to backfill NULL UUIDs with uuid4()
- All Scrobbles now have valid UUIDs for URL resolution

vrobbler/apps/scrobbles/migrations/0104_backfill_null_uuids.py

@@ -0,0 +1,25 @@
+from django.db import migrations
+
+
+def backfill_uuids(apps, schema_editor):
+    Scrobble = apps.get_model("scrobbles", "Scrobble")
+    from uuid import uuid4
+
+    scrobbles = Scrobble.objects.filter(uuid__isnull=True)
+    # Process in batches to avoid loading all into memory
+    while scrobbles.exists():
+        batch = list(scrobbles[:1000])
+        for s in batch:
+            s.uuid = uuid4()
+        Scrobble.objects.bulk_update(batch, ["uuid"], batch_size=1000)
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("scrobbles", "0103_scrobble_species_observation_and_more"),
+    ]
+
+    operations = [
+        migrations.RunPython(backfill_uuids, migrations.RunPython.noop),
+    ]