ActivityPub admin API should resolve site context by ghost_uuid

The ActivityPub admin UI can break when a Ghost site’s configured URL changes, because ActivityPub currently resolves site context from the request Host header.

Example:

1. Ghost is initially configured at https://old.example.com
2. ActivityPub stores site/account state for old.example.com
3. The Ghost site URL is changed to https://new.example.com
4. Admin loads from https://new.example.com/ghost
5. ActivityPub admin API requests are proxied with Host: new.example.com
6. ActivityPub cannot find the existing site row and returns site/auth lookup errors

Example logs:

No site found for 'new.example.com'
No key found for 'new.example.com'

Host-based lookup makes sense for public federation routes, where the URL is part of the ActivityPub actor/object identity. But for authenticated Ghost Admin API requests, the site identity should be stable across URL changes. sites.ghost_uuid seems like the better canonical identifier for admin-context resolution.

Suggested direction:

• Resolve authenticated admin ActivityPub API requests by ghost_uuid, not request host.
• Keep host-based lookup for public federation routes where URL identity matters.
• Handle Ghost URL/domain changes as an explicit actor/domain migration concern, separate from admin API site lookup.

This should prevent the admin UI from breaking after a site URL change while preserving correct federation behavior.

Start by tracing authenticated admin ActivityPub API requests and how they resolve site context from the request Host header. Check how sites.ghost_uuid is available for this lookup, while preserving host-based resolution for public federation routes. Done means admin requests continue to find the site after its URL changes without altering public federation identity behavior.