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/services.py

@@ -173,6 +173,59 @@ def download_file(url, destination):
                     handle.write(chunk)
 
 
+E621_DESCRIPTION_LIMIT = 20000
+
+
+def trim_e621_post(post):
+    """Keep a compact, size-bounded copy of an e621 post payload."""
+    if not isinstance(post, dict):
+        return None
+    file_data = post.get("file") or {}
+    relationships = post.get("relationships") or {}
+    score = post.get("score") or {}
+    categories = post.get("tags") or {}
+    return {
+        "id": post.get("id"),
+        "created_at": post.get("created_at"),
+        "rating": post.get("rating"),
+        "tags": {
+            str(category): [str(tag)[:200] for tag in tags][:500]
+            for category, tags in categories.items()
+            if isinstance(tags, list)
+        },
+        "score": {
+            "up": score.get("up"),
+            "down": score.get("down"),
+            "total": score.get("total"),
+        },
+        "fav_count": post.get("fav_count"),
+        "comment_count": post.get("comment_count"),
+        "sources": [
+            str(source)[:500]
+            for source in (post.get("sources") or [])
+            if source
+        ][:20],
+        "description": str(post.get("description") or "")[:E621_DESCRIPTION_LIMIT],
+        "pools": [
+            int(pool)
+            for pool in (post.get("pools") or [])
+            if str(pool).isdigit()
+        ][:50],
+        "relationships": {
+            "parent_id": relationships.get("parent_id"),
+            "has_children": relationships.get("has_children"),
+        },
+        "file": {
+            "md5": file_data.get("md5"),
+            "ext": file_data.get("ext"),
+            "size": file_data.get("size"),
+            "width": file_data.get("width"),
+            "height": file_data.get("height"),
+        },
+        "uploader_name": post.get("uploader_name"),
+    }
+
+
 def generate_video_thumbnail(md5, path):
     """Extract a JPEG thumbnail from a video, cached under MEDIA_ROOT/thumbs."""
     if not shutil.which("ffmpeg"):