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

frontend/src/features/online/OnlineDetail.tsx

@@ -124,6 +124,7 @@ export function OnlineDetail({ postId }: { postId: number }) {
           post_id: target.id,
           url: target.file.url,
           filename: `${target.file.md5}.${target.file.ext}`,
+          post: target,
         },
       }),
     onSuccess: (item) => {