feat(ai/runner): per-second live runner pricing (remove 720p pixel scaling)

[rickstaa]

What does this pull request do? Explain your changes. (required)

Live runner usage is metered per compute-second on the orchestrator, but the price was run through a hidden fixed-720p@30fps pixel conversion: the converter divided the declared price by 1280*720*30*3600 and the meter multiplied wall-clock seconds by 1280*720*30, so the two 720p factors cancelled to USD/hour x hours and every stream was billed as 720p regardless of real resolution. This makes the operator's price and the /discovery number confusing.

This makes per-second the only behavior: the declared price converts directly to wei/second and usage is metered in wall-clock nanoseconds; /discovery publishes a real wei-per-second rate (PixelsPerUnit = 1e9). The remote signer's LV2V pixel computation also moves to nanoseconds so the consumer side matches the orchestrator's metering. Per-second matches the orchestrator's actual cost basis and the serverless GPU market (Modal/Replicate/RunPod bill per second). Full rationale in #3951.

Specific updates (required)

• newConverterForRunner converts the declared price as USD/second (no 720p scaling).
• convertedPriceInfo publishes wei per nanosecond (PixelsPerUnit = 1e9).
• live_payment_processor meters wall-clock nanoseconds.
• remote_signer LV2V pixel computation now uses nanoseconds to match the orchestrator meter; removed the now-unused defaultSegInfo.
• Tests updated for per-second; signer test ticket EV recalibrated for the nanosecond quantity.

How did you test each of these updates (required)
go build ./..., go test ./ai/runner/ -run LiveRunner, and go test ./server/ -run 'LiveRunner|LivePayment|RemoteSigner' pass. A 10 USD/s price at 2000 USD/ETH converts to 5e15 wei/s, published as {PricePerUnit: 5e15, PixelsPerUnit: 1e9}.

Does this pull request close any open issues?
Fixes #3951

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

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[rickstaa]

@j0sh I just realized that, since this is on the orchestrator side, each orchestrator would need to enable the environment flag, which would be difficult to enforce. New orchestrators would also need to set it explicitly, so switching to this approach does not provide much benefit.

For now, I removed the flag in my latest commit. I think it would be better to coordinate the Daydream gateway migration to the new pricing instead.