Recurring billing: payment mandates + Stripe subscriptions

ADR 0035, Wave 3 — independently shippable before acceptance v2:

• BillingProfile per client: stored payment method / ACH mandate, captured via Stripe on the firm's Connect account (Connect plumbing already built).
• Staff can request a payment method from a client without a proposal.
• Billing consumes subscription.activated → creates a real Stripe subscription; invoices mirror through the proven webhook path (live-proven test-mode capture, 2026-07-05).
• Failed-payment states route to the client billing UI.
  Depends on: P0 Stripe webhook proof (done). Full invoice detail is a separate card.