Design a quota/plan/subscription/invoice data model

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Problem: No quota, plan, subscription, or invoice concept exists anywhere in the codebase today — usage is entirely unmetered from a billing standpoint.

Area: ORB / Billing

Proposal: Design the core data model this whole category depends on — plans, quotas, subscriptions, invoices — before building the individual metering/billing pieces above against it.

Deliverables:

• A written data-model design, reviewed before implementation begins on the other billing items.

Boundaries:

• This is a foundational design task — recommend doing this early within the billing category, even though items aren't strictly ordered across the milestone as a whole.

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JSO-3580

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Sequencing decision: hosted ORB launches free/beta-first with billing deferred (not blocking the ASAP launch). This issue, and the rest of the billing cluster it anchors (#4919 billing SDK, #4930 signup/checkout/plan UI, #4931 onboarding docs, #4932 waitlist/trial/pricing), stay open but are explicitly post-launch — pick this back up once there's a real free-tier user base to convert, not before.

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Deferred as a group tonight (2026-07-19): this cluster (#4919, #4920, #4930, #4931, #4932) needs a real billing-provider and pricing-model decision with no engineering-inferable answer, and nothing else in ORB Cloud Readiness is currently blocked on it. Left open, revisit in a dedicated pricing/business session rather than guessed at here. #4920 (the plan/quota/subscription/invoice data model) is the right one to start with whenever that session happens — the other four build on it.