feat: self-service resume of an existing registration after refresh (S-26, closes #111) (#119)

## What & why

After submitting, the self-service portal held the registration only in in-memory signals, so a **page refresh stranded an in-flight registration** — the reference and its "Documenten aanleveren" / "Trek aanvraag in" actions were lost, with no way back (the reference wasn't in the URL and there was no read endpoint). This is the gap a citizen hit in testing.

Now the portal **resumes on load**:
- **Domain:** `IRegistrationStore.FindOpenByBsnAsync` (the citizen's non-terminal INGEDIEND/IN_BEHANDELING registration) + `GET /registrations/current?bsn=`.
- **BFF:** owner-scoped `GET /self-service/registrations` (bsn from the DigiD token) → the current registration, or **204** when none. Regenerated `services/bff/openapi.json`.
- **Frontend:** `registration-page` calls it on init and restores the submitted view (reference + actions); 204 shows the submit form as before. api-client regenerated (orval).

Closes #111

## Definition of Done

- [x] Linked issue (#111).
- [x] TDD — store `FindOpenByBsnAsync` tests, BFF endpoint tests, an Angular component test (resume-on-load), a Playwright e2e (submit → reload → restored).
- [x] Conventional Commits referencing #111.
- [ ] CI green — validated locally (below); runner CI running.
- [x] `docker compose up` reaches green health — fresh stack + full e2e (3 specs) green.
- [x] Docs — `docs/synthetic-data.md` (new e2e users).
- [ ] ADR — N/A (follows existing BFF/domain patterns; no boundary change).
- [ ] Demo note — the flow is unchanged for the demo; no new demo-script section (happy to add one if wanted).

## Verified locally

- Unit: Big 141 (+7 store tests), Bff 36 (+3 endpoint tests), all suites green.
- Frontend: 12 self-service component tests (incl. resume-on-load); lint + build green.
- **e2e (fresh CI stack): all 3 specs pass** — `registration`, `resume`, `withdrawal` (29.5s, single worker).
- Mutation: domain **91.04%**, bff **100%** (break 90%). `make lint` clean.

## Notes for reviewers

- **Shared-stack isolation:** resume-on-load restores any open registration for the logged-in bsn, so the self-service e2e specs can no longer share `jan-burger` (the verify-* API checks submit as `jan-burger`/`123456782` before the e2e). Each spec now has its own DigiD citizen (`emma`/`sanne`/`lars`-burger); `jan-burger` stays the documented citizen for the verify checks. This is the fix for the two intermittent e2e failures seen during development.
- **Scope:** resumes the current **in-flight** registration only (terminal ones aren't resumed), per the issue's out-of-scope note.

Reviewed-on: #119

libs/api-client/src/lib/generated/bff-api.ts

@@ -24,6 +24,11 @@ import {
   Observable
 } from 'rxjs';
 
+export interface CurrentRegistration {
+  registrationId: string;
+  status: string;
+}
+
 export interface DecideRequest {
   besluit: string;
 }
@@ -200,6 +205,37 @@ export class BffApiV1Service {
     );
   }
 
+ getSelfServiceRegistrations<TData = CurrentRegistration | void>( options?: HttpClientBodyOptions): Observable<TData>;
+ getSelfServiceRegistrations<TData = CurrentRegistration | void>( options?: HttpClientEventOptions): Observable<HttpEvent<TData>>;
+ getSelfServiceRegistrations<TData = CurrentRegistration | void>( options?: HttpClientResponseOptions): Observable<AngularHttpResponse<TData>>;
+  getSelfServiceRegistrations<TData = CurrentRegistration | void>(
+     options?: HttpClientObserveOptions): Observable<TData | HttpEvent<TData> | AngularHttpResponse<TData>> {
+    if (options?.observe === 'events') {
+      return this.http.get<TData>(
+      `/self-service/registrations`,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'events',
+      }
+    );
+    }
+
+    if (options?.observe === 'response') {
+      return this.http.get<TData>(
+      `/self-service/registrations`,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'response',
+      }
+    );
+    }
+
+    return this.http.get<TData>(
+      `/self-service/registrations`,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'body',
+      }
+    );
+  }
+
  postSelfServiceRegistrationsIdWithdraw<TData = void>(id: string, options?: HttpClientBodyOptions): Observable<TData>;
  postSelfServiceRegistrationsIdWithdraw<TData = void>(id: string, options?: HttpClientEventOptions): Observable<HttpEvent<TData>>;
  postSelfServiceRegistrationsIdWithdraw<TData = void>(id: string, options?: HttpClientResponseOptions): Observable<AngularHttpResponse<TData>>;