API site filter

Where Statamic's REST API or GraphQL exposes forms, their form listings take an optional site filter, which lists only the forms assigned to that site, together with the forms that have no sites.

Surface	Parameter	Example
REST, GET /api/forms	site query parameter	/api/forms?site=es
GraphQL, forms query	site argument	forms(site: "es")

curl 'https://example.com/api/forms?site=es'

{
  forms(site: "es") {
    handle
    title
  }
}

Behavior

• Without the filter, or with site: null in GraphQL, the listings are Statamic's.
• A site that names no configured site is an error rather than an empty list: the REST API answers 422 with a message, and GraphQL answers a validation error.
• The filter narrows only the forms the API already exposes.
• The single-form endpoint and query are unchanged.
• Submissions are not filtered.