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.

The call is idempotent per wallet: requesting a delegation that already
exists returns the existing row with created: false and 200, while a
newly recorded request returns 201.

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.