feat(acl): diploma upload stored in the ZGW Documenten API (S-10b, closes #103) (#108)

## What & why

S-10b: the self-service **diploma upload** is now real. After submitting, the citizen picks a PDF and
uploads it; the portal base64-encodes it client-side → BFF → domain → **ACL**, which stores it in the
ZGW **Documenten (DRC) API** as an `enkelvoudiginformatieobject` and relates it to the zaak, then the
`WachtOpDocumenten` wait completes and the case advances to beoordeling. Per §8.1 only the ACL talks to
ZGW.

Closes #103

Mechanism in **ADR-0018** (proposal #107). Builds on S-10a (#102). The zaak-close-on-expiry item is
carved to **#106 (S-10c)**.

## Definition of Done

- [x] Linked Gitea issue (above).
- [x] Failing test committed before the implementation (red→green per layer).
- [x] Conventional Commits referencing the issue (`refs #103`).
- [ ] CI green — all Gitea Actions jobs (pending on this PR).
- [x] `docker compose up` health unaffected (ACL boots on a placeholder informatieobjecttype URL; the real one is injected by verify-domain).
- [x] Docs updated (ADR-0018, demo-script, BACKLOG + S-10c).
- [x] ADR added (`docs/architecture/adr-0018-diploma-upload-via-acl-documenten.md`).
- [x] Demo note in `docs/demo-script.md`.

## Notes for reviewers

- **ACL** (`OpenZaakGateway.StoreDocumentAsync` + `AclService.StoreDiplomaAsync` + `POST /documenten`) reuses the existing gateway patterns (ZGW Bearer, buffered non-chunked body, **no CRS** — Documenten isn't geo). Unit-tested via the stub handler; an **integration test** stores a real document against live OpenZaak (verify-acl).
- **Transport:** base64 JSON on every hop (portal encodes client-side) — I deviated from proposal #107's multipart to keep one contract shape and avoid `IFormFile`/antiforgery/multipart-client plumbing; fine at diploma size (ADR-0018 §Alternatives).
- **Infra:** `seed_catalogus.py` seeds + publishes a "Diploma" `informatieobjecttype` and relates it to the zaaktype (while both concept); `verify-domain` injects its URL into the ACL. No new ZGW scopes (seed applicatie has `heeft_alle_autorisaties`).
- **e2e:** uploads a real PDF (`setInputFiles`) after the openbaar INGEDIEND row confirms the zaak is open (so storage doesn't race the OpenZaak worker).
- **Scope boundary:** the ZGW zaak is not set to a cancellation status on 30-day expiry — that's #106 (S-10c).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #108

services/bff/Bff.Tests/SelfServiceEndpointTests.cs

@@ -114,7 +114,17 @@ public class SelfServiceEndpointTests
 
     private static HttpRequestMessage ProvideDocuments(string? bearer, string id = "reg-123")
     {
-        var request = new HttpRequestMessage(HttpMethod.Post, $"/self-service/registrations/{id}/documents");
+        var request = new HttpRequestMessage(HttpMethod.Post, $"/self-service/registrations/{id}/documents")
+        {
+            // The portal base64-encodes the file client-side and posts it as JSON (S-10b); the bsn is
+            // never in the body — it comes from the DigiD token.
+            Content = JsonContent.Create(new
+            {
+                contentBase64 = Convert.ToBase64String([1, 2, 3]),
+                fileName = "diploma.pdf",
+                contentType = "application/pdf",
+            }),
+        };
         if (bearer is not null)
             request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", bearer);
         return request;
@@ -132,14 +142,19 @@ public class SelfServiceEndpointTests
     }
 
     [Fact]
-    public async Task Provides_documents_for_the_callers_registration_forwarding_the_id_and_bsn()
+    public async Task Provides_documents_for_the_callers_registration_forwarding_id_bsn_and_file()
     {
         using var factory = new BffFactory();
 
         var response = await factory.CreateClient().SendAsync(ProvideDocuments(TestTokens.Valid("123456782"), "reg-9"));
 
         Assert.Equal(HttpStatusCode.NoContent, response.StatusCode);
-        Assert.Equal(("reg-9", "123456782"), factory.Domain.DocumentsProvidedFor);
+        var provided = factory.Domain.DocumentsProvidedFor;
+        Assert.NotNull(provided);
+        Assert.Equal("reg-9", provided!.Value.RegistrationId);
+        Assert.Equal("123456782", provided.Value.Bsn);
+        Assert.Equal(Convert.ToBase64String([1, 2, 3]), provided.Value.ContentBase64);
+        Assert.Equal("diploma.pdf", provided.Value.FileName);
     }
 
     [Fact]