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/account/AccountPage.tsx

@@ -230,11 +230,16 @@ export default function AccountPage() {
       <header>
         <h1 className="text-lg font-semibold">Account</h1>
         {user ? (
-          <p className="mt-1 flex items-center gap-2 text-sm text-ctp-overlay0">
+          <p className="mt-1 flex flex-wrap items-center gap-2 text-sm text-ctp-overlay0">
             <span>Signed in as {user.username}.</span>
             <span className="rounded-full border border-ctp-surface1 px-2 py-0.5 font-mono text-[11px] capitalize text-ctp-subtext0">
               {user.role}
             </span>
+            {user.is_superuser ? (
+              <span className="rounded-full border border-ctp-red/40 bg-ctp-red/10 px-2 py-0.5 font-mono text-[11px] text-ctp-red">
+                admin
+              </span>
+            ) : null}
           </p>
         ) : null}
       </header>