Integrate a billing SDK

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Problem: No billing SDK (payment processing, subscription management) exists in any workspace today.

Area: ORB / Billing

Proposal: Integrate a billing provider, informed by whatever payment/economics decisions are made for the hosted product generally.

Deliverables:

• A working billing integration, even if initially scoped to a single simple plan.

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JSO-3579

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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.

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Note on AI-cost assumptions (2026-07-20): same finding as the comment just added to #4790 — budget this billing SDK integration against Commercial API metered costs, not a Claude Code/Codex subscription. Anthropic actively enforces (as of 2026-04-04) a restriction on third-party/automated tools riding a Pro/Max subscription for anything beyond individual personal use; a paying tenant's usage on a hosted product doesn't qualify. Worth confirming current Commercial/Business Terms directly before this becomes load-bearing for the billing model.

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Scope-tightening note (2026-07-21): this issue's deliverable text ('a working billing integration, even if initially scoped to a single simple plan') doesn't explicitly name invoice generation, receipt delivery, or billing-provider webhook handling (payment succeeded/failed, subscription updated/canceled) as requirements. Per this repo's own gate discipline (it only enforces what an issue explicitly states), whoever picks this up should treat those as required unless this issue's Deliverables get updated to state otherwise — flagging so it isn't silently dropped.