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/acl/Acl.Tests/OpenZaakGatewayTests.cs

@@ -432,4 +432,114 @@ public class OpenZaakGatewayTests
         b64 = (b64.Length % 4) switch { 2 => b64 + "==", 3 => b64 + "=", _ => b64 };
         return Encoding.UTF8.GetString(Convert.FromBase64String(b64));
     }
+
+    // --- StoreDocumentAsync (diploma upload / S-10b) ---
+
+    private static readonly Uri Informatieobjecttype =
+        new("http://openzaak/catalogi/api/v1/informatieobjecttypen/dip");
+
+    private static DocumentRequest SampleDocument(byte[]? inhoud = null) => new(
+        Bronorganisatie: "517439943",
+        Informatieobjecttype: Informatieobjecttype,
+        Vertrouwelijkheidaanduiding: "openbaar",
+        Zaak: new Uri(ZaakUrl),
+        Creatiedatum: new DateOnly(2026, 6, 4),
+        Titel: "Diploma",
+        Auteur: "zorgprofessional",
+        Taal: "nld",
+        Bestandsnaam: "diploma.pdf",
+        Formaat: "application/pdf",
+        Inhoud: inhoud ?? [1, 2, 3, 4]);
+
+    // Routes the two document calls: POST /enkelvoudiginformatieobjecten (documenten) then
+    // POST /zaakinformatieobjecten (zaken).
+    private static StubHandler DocumentStub(Recorder rec) => new(async req =>
+    {
+        rec.Requests.Add(req);
+        rec.ContentLengths.Add(req.Content?.Headers.ContentLength);
+        rec.Bodies.Add(req.Content is null ? null : await req.Content.ReadAsStringAsync());
+
+        return req.RequestUri!.ToString().Contains("/enkelvoudiginformatieobjecten")
+            ? Json(HttpStatusCode.Created, """{"url":"http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten/doc-1"}""")
+            : Json(HttpStatusCode.Created, """{"url":"http://openzaak/zaken/api/v1/zaakinformatieobjecten/rel-1"}""");
+    });
+
+    [Fact]
+    public async Task Storing_a_document_creates_the_informatieobject_then_relates_it_to_the_zaak()
+    {
+        var rec = new Recorder();
+
+        var url = await Gateway(DocumentStub(rec)).StoreDocumentAsync(SampleDocument([10, 20, 30]));
+
+        Assert.Equal("http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten/doc-1", url.ToString());
+
+        // 1. Create the enkelvoudiginformatieobject in the Documenten API.
+        var create = rec.Sent("/enkelvoudiginformatieobjecten");
+        Assert.Equal(HttpMethod.Post, create.Request.Method);
+        Assert.Equal("http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten",
+            create.Request.RequestUri!.ToString());
+        Assert.Equal("Bearer", create.Request.Headers.Authorization!.Scheme);
+        Assert.Contains("\"bronorganisatie\":\"517439943\"", create.Body);
+        Assert.Contains("\"informatieobjecttype\":\"http://openzaak/catalogi/api/v1/informatieobjecttypen/dip\"", create.Body);
+        Assert.Contains("\"creatiedatum\":\"2026-06-04\"", create.Body);
+        Assert.Contains("\"titel\":\"Diploma\"", create.Body);
+        Assert.Contains("\"auteur\":\"zorgprofessional\"", create.Body);
+        Assert.Contains("\"taal\":\"nld\"", create.Body);
+        Assert.Contains("\"bestandsnaam\":\"diploma.pdf\"", create.Body);
+        Assert.Contains("\"formaat\":\"application/pdf\"", create.Body);
+        Assert.Contains("\"vertrouwelijkheidaanduiding\":\"openbaar\"", create.Body);
+        Assert.Contains("\"status\":\"definitief\"", create.Body);
+        // indicatieGebruiksrecht must be set explicitly (false = no usage restrictions); left null,
+        // OpenZaak refuses to close the zaak this document is related to ("indicatiegebruiksrecht-unset").
+        Assert.Contains("\"indicatieGebruiksrecht\":false", create.Body);
+        // The file content is base64-encoded into `inhoud`, with its byte length in `bestandsomvang`.
+        Assert.Contains($"\"inhoud\":\"{Convert.ToBase64String([10, 20, 30])}\"", create.Body);
+        Assert.Contains("\"bestandsomvang\":3", create.Body);
+
+        // 2. Relate that informatieobject to the zaak (Zaken API — no CRS).
+        var relate = rec.Sent("/zaakinformatieobjecten");
+        Assert.Equal(HttpMethod.Post, relate.Request.Method);
+        Assert.Equal("http://openzaak/zaken/api/v1/zaakinformatieobjecten",
+            relate.Request.RequestUri!.ToString());
+        Assert.Contains($"\"zaak\":\"{ZaakUrl}\"", relate.Body);
+        Assert.Contains("\"informatieobject\":\"http://openzaak/documenten/api/v1/enkelvoudiginformatieobjecten/doc-1\"", relate.Body);
+    }
+
+    [Fact]
+    public async Task Storing_a_document_buffers_the_body_and_sends_no_crs_headers()
+    {
+        // uwsgi rejects a chunked body (Content-Length must be present); the Documenten API is not a
+        // geo API, so no CRS headers (unlike the Zaken zaak-create).
+        var rec = new Recorder();
+
+        await Gateway(DocumentStub(rec)).StoreDocumentAsync(SampleDocument());
+
+        var create = rec.Sent("/enkelvoudiginformatieobjecten");
+        Assert.NotNull(create.Length);
+        Assert.True(create.Length > 0);
+        Assert.False(create.Request.Headers.Contains("Accept-Crs"));
+        Assert.False(create.Request.Content!.Headers.Contains("Content-Crs"));
+    }
+
+    [Fact]
+    public async Task Storing_a_document_surfaces_an_openzaak_rejection()
+    {
+        var handler = new StubHandler(_ =>
+            Task.FromResult(new HttpResponseMessage(HttpStatusCode.BadRequest)
+            {
+                Content = new StringContent("""{"detail":"bad"}""", Encoding.UTF8, "application/json"),
+            }));
+
+        var ex = await Assert.ThrowsAsync<HttpRequestException>(
+            () => Gateway(handler).StoreDocumentAsync(SampleDocument()));
+        Assert.Contains("bad", ex.Message);
+    }
+
+    [Fact]
+    public async Task Storing_a_document_rejects_a_null_request()
+    {
+        var handler = new StubHandler(_ => throw new InvalidOperationException("should not be sent"));
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => Gateway(handler).StoreDocumentAsync(null!));
+    }
 }