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/models.py

@@ -20,6 +20,8 @@ class MediaItem(models.Model):
     )
     # Guests never see items whose tags hit the mirrored default blacklist.
     hidden_from_guests = models.BooleanField(default=False, db_index=True)
+    e621_post_id = models.IntegerField(null=True, blank=True, db_index=True)
+    e621_data = models.JSONField(null=True, blank=True)
     created_at = models.DateTimeField(auto_now_add=True)
     updated_at = models.DateTimeField(auto_now=True)
 
@@ -30,9 +32,12 @@ class MediaItem(models.Model):
         return f"J-{self.pk} ({self.md5})"
 
     def save(self, *args, **kwargs):
-        from .guest_filter import item_hidden_from_guests
+        from .guest_filter import item_is_hidden_for_guests
 
-        self.hidden_from_guests = item_hidden_from_guests(self.tags)
+        self.hidden_from_guests = item_is_hidden_for_guests(self)
+        update_fields = kwargs.get("update_fields")
+        if update_fields is not None:
+            kwargs["update_fields"] = set(update_fields) | {"hidden_from_guests"}
         super().save(*args, **kwargs)