Add a cost dimension to product-usage events

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Problem: Existing product-usage tracking covers funnels and retention, not cost — there's no dollar/quota field anywhere in it.

Area: ORB / Billing

Proposal: Add a cost/quota field to this existing event stream, or build a clearly-scoped separate billing-specific stream if mixing concerns there turns out to be the wrong call.

Deliverables:

• A cost-aware usage-event stream, wherever it ends up living.

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JSO-3578