Proposal acceptance v2: sign + pay in one ceremony

The market audit's biggest competitive move (ADR 0035, Wave 4 — deliberately last because it composes everything below it):

Accepting a proposal becomes one ceremony that captures BOTH the e-signature (proposal = engagement letter, rendered from a document template — Wave 2) and a payment method/ACH mandate (Wave 3 BillingProfile), then emits subscription.activated → real Stripe subscription. The engagement moves through a new accepting state and is finalized by the Documenso document.completed webhook, with the signed letter landing as a delivered document (Wave 1).

Both vendor paths are now live-proven (Documenso e-sign on prod; Stripe signed webhook). Risk note recorded and accepted 2026-07-05: this widens launch scope; proofs stayed P0 and this build starts only after the waves under it.