feat(portal): 'Documenten aanleveren' action on the self-service page (refs #102)

After submitting, the zorgprofessional supplies their documents; the page posts to
the BFF keyed by the reference and confirms ("Uw documenten zijn aangeleverd"), with
a surfaced failure + retry. The registration e2e provides documents before the
behandelaar step, since the process now parks at WachtOpDocumenten first.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/e2e/registration.spec.ts

@@ -25,6 +25,12 @@ test('DigiD submit → public INGEDIEND → behandelaar goedkeurt → public ING
   const reference = (await confirmation.textContent())?.match(/Referentie:\s*([0-9a-fA-F-]+)/)?.[1];
   expect(reference, 'the confirmation shows a registration reference').toBeTruthy();
 
+  // Provide the documents the registration is waiting for (S-10a): the process parks at
+  // WachtOpDocumenten right after the zaak is opened, so it only reaches the behandelaar's werkbak once
+  // the documents are supplied. (S-10b turns this into a real file upload; here it is the trigger.)
+  await page.getByRole('button', { name: /documenten aanleveren/i }).click();
+  await expect(page.getByText(/documenten zijn aangeleverd/i)).toBeVisible();
+
   // The openbaar register (anonymous, its own origin) shows the submitted entry once the projection
   // catches up. The projection updates asynchronously (NRC → event-subscriber), and the register loads
   // on open, so reload until *this* submission's row appears. We poll on the reference cell (not a