Create MPP deposit intent (service key, guest checkout)

Service-key only (the same callers as POST /service/fulfillments: enforcer-mp-merchant, enforcer-airdrop). The guest-checkout twin of POST /me/fulfillments/{id}/mpp-deposit-intent: a service-created fulfillment has the payee as creator and no sender, so no account can raise its intent. Here the fulfillment must belong to X-Tenant-ID; any other tenant's fulfillment is 404 (existence is not revealed). Otherwise identical: one intent PER PAYER, idempotent on (fulfillment, refund_address) (repeat = 200 with that payer's intent; a different refund_address gets its own intent and forwarder; at most 10 payers per fulfillment, then 429 too_many_intents; a fulfillment no longer pending or claimed is 409 fulfillment_not_payable), ref is the fulfillment memo (409 fulfillment_memo_invalid otherwise), refund_address required and committed into the CREATE2 forwarder, notional above 10,000 USDC refused. Pay Base only, MPP only, the exact amount_to_send, before expiry.