Add to favourites (idempotent; 409 FAVORITES_CAP at 10 per type)

The site key identifying which website this request belongs to. May be omitted only when the request comes from a domain already verified for the site (the Origin/Host then resolves it), or when the deprecated X-Subdomain header is used.

Publishable (pk_…) or secret (sk_…) key for this site. Required on writes; accepted on reads. NOTE: currently rolling out in monitor mode — a write without a valid key is logged rather than refused, so this is documented as optional. It becomes required on writes when enforcement is switched on; send it now.