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/views.py

@@ -223,6 +223,7 @@ class DownloadToLibraryView(APIView):
         url = str(request.data.get("url") or "").strip()
         post_id = request.data.get("post_id")
         filename = str(request.data.get("filename") or "").strip()
+        post_payload = request.data.get("post")
         if not url.startswith(("http://", "https://")):
             return Response(
                 {"detail": "A valid file URL is required."},
@@ -244,9 +245,26 @@ class DownloadToLibraryView(APIView):
                 {"detail": f"Download failed: {exc}"},
                 status=status.HTTP_400_BAD_REQUEST,
             )
+
+        update_fields = []
         if item.uploaded_by_id is None:
             item.uploaded_by = request.user
-            item.save(update_fields=["uploaded_by", "updated_at"])
+            update_fields.append("uploaded_by")
+
+        trimmed = services.trim_e621_post(post_payload)
+        if trimmed is not None:
+            if trimmed.get("id") is not None:
+                item.e621_post_id = int(trimmed["id"])
+            elif post_id and str(post_id).isdigit():
+                item.e621_post_id = int(post_id)
+            item.e621_data = trimmed
+            update_fields += ["e621_post_id", "e621_data"]
+            if not item.rating and trimmed.get("rating") in {"s", "q", "e"}:
+                item.rating = trimmed["rating"]
+                update_fields.append("rating")
+
+        if update_fields:
+            item.save(update_fields=update_fields + ["updated_at"])
         return Response(
             MediaItemSerializer(item, context={"request": request}).data,
             status=status.HTTP_201_CREATED,