Archive the expense

As the app's Archive. The expense drops out of default lists and totals (filter[archived] brings it back), isn't reimbursed by auto-payment or synced to accounting, and is read-only: other writes to it answer 409 until it is unarchived. A pending approval is cancelled, which returns the expense to draft. An expense whose reimbursement is scheduled, in progress or paid can't be archived (422). Idempotent: archiving an archived expense returns it unchanged. Needs the role's Expenses Archive permission (or the expense to be the token user's own), and expense archiving turned on for the account.