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/domain/Big.Api/Program.cs

@@ -120,8 +120,17 @@ app.MapPost("/registrations/{id}/documents", async (string id, ProvideDocumentsR
 
     if (string.IsNullOrWhiteSpace(body?.Bsn))
         return Results.BadRequest(new { error = "A bsn is required to provide documents." });
+    if (string.IsNullOrWhiteSpace(body.ContentBase64))
+        return Results.BadRequest(new { error = "A document is required." });
 
-    var outcome = await provide.HandleAsync(new ProvideDocumentsCommand(new RegistrationId(guid), body.Bsn), ct);
+    byte[] content;
+    try { content = Convert.FromBase64String(body.ContentBase64); }
+    catch (FormatException) { return Results.BadRequest(new { error = "The document content is not valid base64." }); }
+
+    var command = new ProvideDocumentsCommand(
+        new RegistrationId(guid), body.Bsn, content,
+        body.FileName ?? "diploma.pdf", body.ContentType ?? "application/pdf");
+    var outcome = await provide.HandleAsync(command, ct);
     return outcome == ProvideDocumentsOutcome.Accepted ? Results.NoContent() : Results.NotFound();
 });
 
@@ -152,7 +161,7 @@ public sealed record DecideRequest(string Besluit);
 
 public sealed record WithdrawRequest(string Bsn);
 
-public sealed record ProvideDocumentsRequest(string Bsn);
+public sealed record ProvideDocumentsRequest(string Bsn, string ContentBase64, string? FileName = null, string? ContentType = null);
 
 public sealed record RegistrationResponse(string RegistrationId, string Status, string? ZaakUrl);