Enrol a site in a proposition.

This is the commitment point — it activates the proposition for the site.

Idempotent: if the site is already enrolled, returns already_enrolled without making changes.

A 200 response always means the site is enrolled; status only distinguishes a new enrolment from an existing one. Any failure returns an HTTP error and leaves the site unenrolled, so fix the cause and call again.

If the site previously withdrew from this proposition (for example via /unenrol), returns 422 with detail.code set to previously_withdrew. Set override_withdrawal to true to re-enrol; this is the recommended default for user-initiated re-enrolment.

{
  "proposition": "limited_pause"
}

{
  "site_id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
  "status": "enrolled"
}