Fully replace an existing plan by ID

Replaces the entire plan record identified by {id} with the supplied payload. This is a full replacement — fields omitted from data are not preserved from the prior version, so submit the complete desired plan data on every call, not a partial diff. Submitting the same payload twice for the same {id} is idempotent. The caller must hold the UPDATE_PLAN permission. As with plan creation, data.payer is extracted and stored as payerId, and the referenced payer is not validated for existence. data's full shape is defined by the external Plan JSON schema and may include required fields beyond payer.