Keep e621 metadata on downloaded posts; clear up account roles

Roles:
- JakeBreathild is now staff + superuser (the real account); the 'jake'
  smoke-test account was demoted to a regular user
- /me exposes is_superuser and the account page shows an admin badge

e621 metadata:
- MediaItem gains e621_post_id and e621_data (trimmed post payload:
  tags by category, rating, score, favourites, comments, sources,
  description, pools, relationships, file info, uploader)
- Download to Library accepts the post payload from the SPA and stores
  it; the item's custom rating is seeded from the e621 rating when empty
- Library detail shows an e621 metadata card: link to the in-app post,
  score/favourites/comments, taxonomy-coloured tags, DText description,
  sources and pools; grid cards get an e621 badge and fall back to the
  e621 rating for their colour (display_rating)
- Guest visibility now also considers e621 tags, so downloaded explicit
  content is hidden from anonymous visitors

backend/apps/library/migrations/0003_mediaitem_e621_data_mediaitem_e621_post_id.py

@@ -0,0 +1,23 @@
+# Generated by Django 6.1.1 on 2026-09-17 15:25
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('library', '0002_mediaitem_hidden_from_guests_mediaitem_uploaded_by'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='mediaitem',
+            name='e621_data',
+            field=models.JSONField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name='mediaitem',
+            name='e621_post_id',
+            field=models.IntegerField(blank=True, db_index=True, null=True),
+        ),
+    ]