Record that the account needs a delegation for one of its linked wallets.

The body is optional. With no party_id the trading wallet is chosen,
falling back to the RFQ wallet when that is the only one linked. Supply
party_id to target a specific linked wallet.

A new request returns 201 with created: true. An existing expired or
revoked row is renewed in place: its status becomes requested, its old
delegation_cid is cleared to NULL, any pending revocation flag is cleared,
and updated_at is refreshed. This also returns 201 with created: true,
keeping the row ID and created_at.

Other existing statuses (requested, waiting, ready) are
returned unchanged with created: false and 200. Repeated requests
therefore do not restart a live proposal. Only a new or renewed request
creates a delegation.requested audit entry.

A new row starts at requested — this records the intent, it does not
produce a signed delegation. Poll GET /api/trading/delegation to follow it
to ready.