Make Cloud config and billing optional for self-hosters #994

Parent

Spec: 1.0 tree cleanup and release contracts

What to build

Enforce the agreed required Cloud environment, remove dead overrides and fallbacks, and implement the Polar-disabled behavior decided in #977: one purchasable plan (Free and Teams dropped, no signup-time Polar call, no plan column), billing nav/route hidden and Inngest billing functions unregistered when self-hosted, webhook left 404.

Acceptance criteria

• Cloud config tests cover missing required variables and a valid Polar-free startup.
• No installer override, Variable Groups flag, or Inngest signing-key fallback survives; billing checks and UI follow the Polar-optional decision.

Blocked by

• Remove retired Cloud authoring and capability paths
• Polar-optional Cloud: what 'billing disabled' means #977

Plan copy decided in Which plans Cloud sells at 0.2.0: the hosted Billing card reads Pro · $9/mo · custom domains on Services. No second plan.