feat(zgw): OpenZaak create-zaak, first write slice (WP-50)

Extends the IZaakSource seam (WP-49, read-only) with CreateZaak: submitting
an aanvraag now also registers a Zaak + Status + Rol in OpenZaak when
Zgw:Enabled=true, routed through the existing /applications/{id}/submit
endpoint with the FE response DTO unchanged (ADR-0001/ADR-0005 — the
endpoint never branches on the config flag itself, DI already picked the
implementation).

- ZgwOptions gains a Type→zaaktype-URL map + the two RSINs a Zaak needs.
- LocalZaakSource.CreateZaak is a pure passthrough of what the endpoint
  already computes locally (zero behaviour change for the offline default).
- OpenZaakZaakSource.CreateZaak POSTs the zaak (identificatie = the same
  local reference, so both stay in sync), resolves + POSTs the initial
  status and the initiator rol (BSN) via Catalogi lookups, and maps the
  result back into the submit response.
- Marked ponytail shortcuts: first-statustype/roltype-Catalogi-returns
  (no per-type config) and no compensating transaction on partial failure
  — both fine for a first slice against a demo backend.

Verified: full `npm run ci` green, zero api-client drift, 144/144 backend
tests (142 existing + 2 new stub-handler tests asserting the POST bodies
+ type→zaaktype mapping per the acceptance criteria).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/BigRegister.Api/Program.cs

@@ -299,7 +299,7 @@ api.MapDelete("/applications/{id}", (string id) =>
 
 // Submit runs the server-owned rules, sets autoApprovable, and transitions the
 // aanvraag. handmatig no longer 422s (ADR-0002): it becomes a manual (pending) case.
-api.MapPost("/applications/{id}/submit", (string id, SubmitApplicationRequest req, HttpContext ctx) =>
+api.MapPost("/applications/{id}/submit", (string id, SubmitApplicationRequest req, HttpContext ctx, IZaakSource zaken) =>
 {
   var existing = ApplicationStore.Get(id, DocumentStore.DemoOwner);
   if (existing is null) return Results.NotFound();
@@ -324,7 +324,13 @@ api.MapPost("/applications/{id}/submit", (string id, SubmitApplicationRequest re
   app.Logger.LogInformation(
       "aanvraag submit id={Id} type={Type} outcome={Outcome} auto={Auto} reference={Reference}",
       id, existing.Type, reject is null ? "accepted" : "rejected", autoApprovable, submitted.Referentie);
-  return Results.Ok(new SubmitApplicationResponse(submitted.Referentie!, submitted.ToStatusDto(DateTimeOffset.UtcNow)));
+
+  // WP-50: route the create through the IZaakSource seam — LocalZaakSource is a passthrough
+  // of what was computed above; OpenZaakZaakSource (Zgw:Enabled=true) also registers a zaak
+  // in OpenZaak and maps its result back into this same response shape (ADR-0001/ADR-0005:
+  // zero FE contract change either way).
+  var (referentie, status) = zaken.CreateZaak(submitted, DateTimeOffset.UtcNow);
+  return Results.Ok(new SubmitApplicationResponse(referentie, status));
 })
 .Produces<SubmitApplicationResponse>()
 .ProducesProblem(StatusCodes.Status409Conflict)