Invoice write surface.

Exposes five user-path invoice actions on Accrue.Billing via a defdelegate facade: finalize_invoice, void_invoice, pay_invoice, mark_uncollectible, send_invoice.

Each action follows a consistent pattern: call the Stripe API, decompose the response into local schema changes via InvoiceProjection, write the updated invoice row and upsert its line items, and record an audit event — all in a single database transaction.

pay_invoice/2 returns an intent result ({:ok, %Invoice{}} or {:ok, :requires_action, pi}) because Stripe may surface SCA/3DS. The other four actions return plain {:ok, %Invoice{}}. Every action has a bang variant that raises on {:error, _}; pay_invoice!/2 additionally raises Accrue.ActionRequiredError on :requires_action.

The webhook path uses the force-status bypass on the Invoice schema — that bypass is NOT reachable from this module. Illegal user-path transitions (e.g. draft -> paid) are rejected by Invoice.changeset/2 with an error on :status and propagate as {:error, %Ecto.Changeset{}}.