Model-ID usage attribution - add model_id to create_signed_ticket + sign RemotePaymentState per-model

[eliteprox]

Security + accuracy. model_id missing from create_signed_ticket, so usage can't be attributed per model; a caller could route usage to a cheaper-advertised model (usage theft/misattribution).

1. go-livepeer: add model_id to create_signed_ticket — draft PR feat(remote_signer): emit model_id in create_signed_ticket Kafka events go-livepeer#3947
2. livepeer-python-gateway: sign RemotePaymentState per-model (PR to open)
3. clearinghouse collector: map model_id into CloudEvents usage dims (ties to Benthos collector — create_signed_ticket → CloudEvents → OpenMeter #7, Provision shared OpenMeter instance with tenant data isolation #12)

Acceptance: model_id flows to OpenMeter usage dims; ticket signed for model A can't be attributed to model B; peer review validates the threat model. Linear: ENG-148.

[linear-code]

ENG-151

ENG-148

[eliteprox]

Closing as completed for the clearinghouse-owned slice of this work, with go-livepeer kept non-blocking per Mehrdad's guidance.

What this issue scoped

1. go-livepeer: emit model_id on create_signed_ticket — livepeer/go-livepeer#3947 (still under review; deliberately not blocking the project)
2. livepeer-python-gateway: sign RemotePaymentState per-model — still outstanding / out of this repo
3. clearinghouse collector: map model_id into CloudEvent usage dims — done (collector.yaml + meter groupBy in catalog.json)

Honest status: the threat-model fix is only fully closed once go-livepeer #3947 merges and the gateway signs per-model. Clearinghouse can already attribute model_id whenever upstream emits it (defaults to "unknown" otherwise). Keeping the go-livepeer PR on the side so Deliverable 2 is not blocked.

[eliteprox]

Follow-up: the remaining upstream pieces are now tracked as side-track issues (label go-livepeer, non-blocking):

• side-track: livepeer-python-gateway — per-model RemotePaymentState + model_id attribution #71 — livepeer-python-gateway per-model RemotePaymentState / model_id attribution (pairs with go-livepeer#3947, gateway PR #33)
• side-track: livepeer-python-gateway — fail fast on signer HTTP 483 insufficient balance #72 — livepeer-python-gateway fail-fast on signer HTTP 483 (PR #44)