feat(workflow): diploma-eligibility DMN routes foreign diplomas via CBGV-advies (S-13, closes #14) (#101)

## What & why

S-13: a diploma's origin decides its route. A **DMN** (`diploma-eligibility`) is evaluated inline by
the registratie process as a **`businessRuleTask`**; an exclusive gateway routes a **foreign**
(Buitenlands) diploma through a new **CBGVAdvies** user task before `Beoordelen`, a **domestic** one
straight there (PRD flow 4). The domain's only new job is carrying the diploma origin and passing it
as a process start variable.

Chose **Option B (DMN in the BPMN)** over the issue's literal "evaluated by the Domain Service via
Workflow Client" wording — keeps the decision a first-class workflow artefact and §8.2 clean.
Rationale in **ADR-0016** (proposal #100); noted on this issue.

Closes #14

## Definition of Done

- [x] Linked Gitea issue (above).
- [x] Failing test committed before the implementation.
- [x] Implementation makes the test pass.
- [x] Conventional Commits referencing the issue (`refs #14`).
- [ ] CI green — all Gitea Actions jobs.
- [x] `docker compose up` from a fresh clone reaches green health checks within 3 minutes (additive; DMN deployed by flowable-init).
- [x] Docs updated (ADR-0016, demo note).
- [x] ADR added (`docs/architecture/adr-0016-diploma-eligibility-dmn.md`).
- [x] Demo note in `docs/demo-script.md`.

## How it was built (TDD)

- **Domain**: `DiplomaOrigin` on the aggregate + submit command; threaded through the process-start port so the Workflow Client emits a `diplomaOrigin` start variable. Red → green.
- **DMN + BPMN**: `workflows/diploma-eligibility.dmn` (origin → route); `businessRuleTask` + exclusive gateway + `CBGVAdvies` user task in `registratie.bpmn`; DMN deployed to Flowable's DMN engine by `flowable-init`.
- **Both paths**: `Een diploma op herkomst routeren` acceptance scenarios (origin carried into the process) + unit tests; verify-domain drives a foreign registration through CBGVAdvies→Beoordelen and the domestic one straight to Beoordelen — exercising both DMN branches live.

## Notes for reviewers

- Deviation from the issue's Option-A wording is deliberate and recorded (ADR-0016); the outcome is unchanged.
- The self-service eIDAS→foreign wiring is out of scope here (this slice is area:domain + area:workflow); the domain submit accepts an optional `diplomaOrigin` so the foreign path is drivable.
- Local green: domain unit 109, acceptance 15, `dotnet format`, Release build (0 errors), **domain mutation 95.39%** (break 90). The DMN/`businessRuleTask` REST wiring is CI-verified on verify-stack (no local full-stack run here).

Reviewed-on: #101

docs/demo-script.md

@@ -343,3 +343,42 @@ candidate group behandelaar → teamlead (§8.2).
 > Both branches (escalate after 14 days; no-op when completed in time) are covered by the
 > `Een beoordeling escaleren` acceptance scenarios and the Workflow Client unit tests; the timer firing
 > and reassignment are asserted live by the verify-domain check.
+
+## S-13 — Diploma-eligibility: foreign diplomas route through CBGV-advies (#14, ADR-0016)
+
+A registration's diploma origin decides its route. A DMN service task in the registratie
+process evaluates the `diploma-eligibility` decision on the `diplomaOrigin` start variable: a
+**foreign** (Buitenlands) diploma is routed through an extra **CBGV-advies** user task before
+beoordeling; a **domestic** (Binnenlands) one goes straight to beoordeling. The decision lives in the
+DMN, not in code — a beheerder can read and adjust the decision table directly.
+
+The self-service portal's eIDAS→foreign wiring is a later slice; for now the origin is submitted to
+the domain directly, so the demo drives it through the domain endpoint:
+
+```bash
+# 1. Submit a foreign-diploma registration to the domain (note the returned Location/reference):
+DOM=http://localhost:8080   # domain service
+curl -s -i -X POST "$DOM/registrations" -H 'Content-Type: application/json' \
+  -d '{"bsn":"123456782","diplomaOrigin":"Buitenlands"}' | grep -i '^location:'
+#
+# 2. Once the zaak is opened, the process parks at the CBGV-advies task (NOT Beoordelen). In Flowable:
+FL=http://localhost:8090/flowable-rest/service
+curl -s -u rest-admin:test -X POST "$FL/query/tasks" -H 'Content-Type: application/json' \
+  -d '{"processDefinitionKey":"registratie","taskDefinitionKey":"CBGVAdvies"}' | python3 -m json.tool
+#
+# 3. Complete the CBGV-advies task; the case then advances to the regular Beoordelen task:
+TID=$(curl -s -u rest-admin:test -X POST "$FL/query/tasks" -H 'Content-Type: application/json' \
+  -d '{"processDefinitionKey":"registratie","taskDefinitionKey":"CBGVAdvies"}' \
+  | python3 -c 'import sys,json;print(json.load(sys.stdin)["data"][0]["id"])')
+curl -s -u rest-admin:test -X POST "$FL/runtime/tasks/$TID" \
+  -H 'Content-Type: application/json' -d '{"action":"complete"}'
+# A domestic submission (default, or "Binnenlands") skips CBGV-advies and parks straight at Beoordelen.
+```
+
+**The path:** domain sets the `diplomaOrigin` start variable → registratie process DMN
+DMN service task sets `route` → exclusive gateway → foreign: `CBGVAdvies` user task → `Beoordelen`;
+domestic: `Beoordelen` directly (§8.2, ADR-0016).
+
+> The domestic/foreign paths are covered by the `Een diploma op herkomst routeren` acceptance
+> scenarios and unit tests (the origin is carried into the process); the DMN decision and the
+> foreign→CBGV routing are asserted live by the verify-domain check.