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).

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Reviewed-on: #108

docs/demo-script.md

@@ -434,3 +434,31 @@ routing → `Beoordelen`), OR the `P30D` interrupting timer fires → `Registrat
 > Both branches are covered by the `Een documenttermijn laten verlopen` acceptance scenarios (worker +
 > aggregate) and unit tests; the wait completion and the 30-day timer firing are asserted live by the
 > verify-domain check.
+
+## S-10b — Diploma upload stored in the ZGW Documenten API (#103, ADR-0018)
+
+The self-service "Documenten aanleveren" action (S-10a) is now a **real file upload**: after submitting,
+the citizen picks a PDF and uploads it. The portal base64-encodes the file client-side and posts it to
+the BFF; the BFF forwards it to the domain, which stores it via the **ACL** as a ZGW
+`enkelvoudiginformatieobject` in the **Documenten (DRC) API** and relates it to the zaak — then completes
+the `WachtOpDocumenten` wait so beoordeling can proceed. Per §8.1 only the ACL talks to ZGW.
+
+```bash
+make up
+# 1. Log in as jan-burger / test123, submit, then — once the openbaar register shows the row —
+#    choose a PDF under "Documenten aanleveren" and upload it. The page confirms "aangeleverd".
+open http://localhost:8140
+#
+# 2. Automated: the walking-skeleton e2e now uploads a real PDF before the behandelaar approves.
+make verify-e2e
+#
+# 3. The ACL integration test proves the document is really created in the Documenten API and
+#    related to the zaak (against a live OpenZaak):
+make verify-acl        # → "Storing a diploma creates a real informatieobject related to the zaak"
+```
+
+**The path:** portal (base64) → BFF `POST /self-service/registrations/{id}/documents` → domain
+`ProvideDocuments` → ACL `POST /documenten` → ZGW `enkelvoudiginformatieobjecten` +
+`zaakinformatieobjecten`; the wait is then completed and the case advances to Beoordelen (§8.1, ADR-0018).
+
+> Setting the ZGW zaak to a cancellation status on 30-day expiry is a follow-up (S-10c, #106).