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/serializers.py

@@ -19,6 +19,7 @@ class MediaItemSerializer(serializers.ModelSerializer):
     rating = serializers.ChoiceField(choices=["", "s", "q", "e"], required=False)
     locations = MediaLocationSerializer(many=True, read_only=True)
     j_id = serializers.SerializerMethodField()
+    display_rating = serializers.SerializerMethodField()
     filename = serializers.SerializerMethodField()
     extension = serializers.SerializerMethodField()
     kind = serializers.SerializerMethodField()
@@ -33,6 +34,7 @@ class MediaItemSerializer(serializers.ModelSerializer):
             "md5",
             "size",
             "rating",
+            "display_rating",
             "tags",
             "notes",
             "filename",
@@ -40,6 +42,8 @@ class MediaItemSerializer(serializers.ModelSerializer):
             "kind",
             "uploaded_by",
             "uploaded_by_id",
+            "e621_post_id",
+            "e621_data",
             "locations",
             "created_at",
             "updated_at",
@@ -49,11 +53,14 @@ class MediaItemSerializer(serializers.ModelSerializer):
             "j_id",
             "md5",
             "size",
+            "display_rating",
             "filename",
             "extension",
             "kind",
             "uploaded_by",
             "uploaded_by_id",
+            "e621_post_id",
+            "e621_data",
             "locations",
             "created_at",
             "updated_at",
@@ -62,6 +69,13 @@ class MediaItemSerializer(serializers.ModelSerializer):
     def get_j_id(self, obj):
         return f"J-{obj.id}"
 
+    def get_display_rating(self, obj):
+        if obj.rating:
+            return obj.rating
+        data = obj.e621_data or {}
+        rating = data.get("rating") if isinstance(data, dict) else None
+        return rating if rating in {"s", "q", "e"} else ""
+
     def get_uploaded_by(self, obj):
         return obj.uploaded_by.username if obj.uploaded_by else None