Save AI provider configuration

Saves the providerConfig JSON for the target site. Accepts an optional siteId query parameter (site identifier / UUID, or the literal SYSTEM_HOST). Hostname values are not supported — use the site identifier. When siteId is omitted, saves to the site derived from the HTTP Host header. An unresolvable siteId returns 400. Credential fields set to "*****" are preserved from the existing stored configuration. Requires CMS admin.