feat: exclude API-billed instances from federated usage totals

[atomantic]

Summary

The Across Instances card on the Usage page now lets you turn Subscriptions off for a federated instance that pays real API rates instead of riding this install's plans.

That instance stays listed (the spend is still real) but is left out of the combined fleet total, so an API-billed box no longer inflates the subscription-priced figure.

The choice is machine-local (settings.usageApiBilledInstanceIds) — it is this install's view of which fleet members ride the plans, not a property of the instance itself — and does not ride the usage digest.

Test plan

• Open Usage with at least two federated instances showing under Across Instances
• Confirm the combined total still matches the sum of every row while every Subscriptions switch is on
• Turn Subscriptions off on an API-billed instance
• Confirm the row stays listed with an API billed pill, and the combined total / "N of M instances on subscriptions" copy drop that row
• Reload the page and confirm the exclusion persisted
• Turn the switch back on and confirm the instance counts again
• cd server && npx vitest run services/usageFleetBilling.test.js services/peerUsage.test.js routes/usage.test.js
• cd client && npx vitest run src/components/usage/FleetUsageCard.test.jsx src/pages/UsagePage.test.jsx