side-track: livepeer-python-gateway — per-model RemotePaymentState + model_id attribution

[eliteprox]

Summary

Side-track (non-blocking for clearinghouse deliverables). Completes the gateway half of the model-ID usage-attribution work originally scoped in #41.

Clearinghouse already maps model_id into CloudEvent dims. Full end-to-end threat-model closure still needs:

1. go-livepeer — emit model_id on create_signed_ticket — livepeer/go-livepeer#3947 (under review; kept non-blocking per Mehrdad)
2. livepeer-python-gateway — sign RemotePaymentState per-model / send real capability + model_id so usage cannot be misattributed to a cheaper-advertised model — see livepeer/livepeer-python-gateway#33

Why side-track

Upstream go-livepeer + gateway changes should not block the clearinghouse project. Track here so the relationship stays visible on the milestone without gating D2.

Acceptance

• Gateway signs / pays with the real capability + model_id
• Ticket signed for model A cannot be attributed to model B once go-livepeer#3947 lands
• Peer review validates the threat model (Linear: ENG-148)

Related

• Parent (closed, clearinghouse slice done): Model-ID usage attribution - add model_id to create_signed_ticket + sign RemotePaymentState per-model #41
• go-livepeer: feat(remote_signer): emit model_id in create_signed_ticket Kafka events go-livepeer#3947
• Gateway PR: fix(byoc): send real capability + model_id for usage attribution livepeer-python-gateway#33

[linear-code]

ENG-189

[eliteprox]

Split out from #41 (clearinghouse collector slice closed). Pairs with go-livepeer#3947. Related gateway balance-gate side-track: #72.