Spec: ongoing account-settings/billing-management UI (sibling to #4930's one-time checkout)

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Problem

No ongoing (post-checkout) account-settings/billing-management UI surface is specced anywhere. Exhaustive search across "billing", "checkout", "account settings", "customer portal", "manage subscription", "invoice history", "payment method" turned up nothing. #4930 covers only the one-time signup/checkout/plan-selector flow (initial purchase); #4920 is the backend data model only. Nothing tracks an ongoing self-service page for updating a payment method, viewing invoice history, or changing/canceling a plan.

Area

apps/loopover-ui/, hosted-product onboarding cluster (#4926/#4930/#4931/#4932).

Proposal

Define, at spec level: what an ongoing account-settings surface needs to expose (payment method update, invoice/receipt history, plan change/cancellation) — this is the natural sibling to #4930's one-time checkout flow, currently missing from the tracker entirely.

Deliverables

• A written spec for the ongoing account-settings surface's required capabilities.

Resources

• Build a signup/checkout/plan-selector UI #4930 (one-time checkout — this issue's ongoing counterpart)
• Design a quota/plan/subscription/invoice data model #4920 (backend data model this UI reads/writes)

Boundaries

Spec only — depends on #4919/#4920/#4930 landing first for the underlying billing system to manage.

maintainer-only — product spec, depends on unbuilt billing infrastructure.

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JSO-5415