Download the invoice document (PDF)

Streams the rendered invoice document for the given invoice ("Download PDF" on the invoice detail page and the per-row "Download invoice" action in the Invoices page). The document reflects the invoice's current persisted state (number, period, totals, status); the invoice itself is never mutated by the download. The UI disables the action for CANCELLED invoices ("Cancelled — no PDF"), and the backend mirrors that rule. Validation: invoiceId must reference an invoice visible in the active scope (otherwise 404 NOT_FOUND); the invoice status must permit download — CANCELLED invoices have no document and are rejected with 409 CONFLICT; the caller must hold billing read access in the scope (otherwise 403 FORBIDDEN). Async & state: Synchronous read returning the file stream; no state change and no SSE frames. Document rendering is performed (or served from cache) inline within the request. Errors: 401 unauthenticated; 403 caller lacks billing read access; 404 unknown invoiceId in the scope; 409 invoice is CANCELLED and has no downloadable document; 500 on render/stream failure. Idempotency/Concurrency: Safe, cacheable; no idempotency key required. Repeated downloads return byte-identical documents for an unchanged invoice.