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>

docs/reference/openzaak-integration.md

@@ -1,9 +1,10 @@
 # OpenZaak / ZGW integration — how the BFF connects (& how to extend)
 
-How the BFF sources cases from a real **OpenZaak** (ZGW APIs) while the frontend stays
-unchanged. For the _why_, see [ADR-0005](architecture/0005-openzaak-behind-bff.md); this page
-is _how the seam is built and how to add the next slice_. Built in
-[WP-49](../project/backlog/WP-49-openzaak-zaken-read-seam.md) (read-only zaken).
+How the BFF sources (and now creates) cases against a real **OpenZaak** (ZGW APIs) while the
+frontend stays unchanged. For the _why_, see [ADR-0005](architecture/0005-openzaak-behind-bff.md);
+this page is _how the seam is built and how to add the next slice_. Built in
+[WP-49](../project/backlog/WP-49-openzaak-zaken-read-seam.md) (read-only zaken) and
+[WP-50](../project/backlog/WP-50-openzaak-create-zaak.md) (the first write: create-zaak).
 
 ## The one rule: OpenZaak sits behind the BFF, never in the browser
 
@@ -14,14 +15,48 @@ with **zero frontend change and no api-client drift**.
 
 ## The seam (data source by config)
 
-- `Data/IZaakSource.cs` — the cases READ interface. Returns the existing
-  `ApplicationSummaryDto`, so each implementation owns its own mapping.
+- `Data/IZaakSource.cs` — the cases READ + (WP-50) WRITE interface: `ListCases` and
+  `CreateZaak`. Both return the existing DTOs, so each implementation owns its own mapping.
 - `Data/LocalZaakSource.cs` — **default**; reads the local SQLite `ApplicationStore`
-  (offline, unchanged behaviour).
+  (offline, unchanged behaviour). `CreateZaak` is a pure passthrough of what the submit
+  endpoint already computed locally — no external call.
 - `Zgw/OpenZaakZaakSource.cs` — the OpenZaak client; selected only when `Zgw:Enabled=true`.
+  `CreateZaak` posts a Zaak, then a Status, then a Rol (see below).
 - Wiring (`Program.cs`): `if (Zgw:Enabled) AddHttpClient<IZaakSource, OpenZaakZaakSource>()
-else AddSingleton<IZaakSource, LocalZaakSource>()`. The `/admin/cases` endpoint resolves
-  `IZaakSource` from DI — routes + DTOs untouched.
+else AddSingleton<IZaakSource, LocalZaakSource>()`. The `/admin/cases` GET and the
+  `/applications/{id}/submit` POST both resolve `IZaakSource` from DI — routes + DTOs
+  untouched either way.
+
+## Create-zaak (WP-50) — the first write
+
+`POST /applications/{id}/submit` already persists the aanvraag locally (`ApplicationStore.Submit`
+— unconditionally, regardless of `Zgw:Enabled`, since draft/step/document bookkeeping stays
+local either way) and only THEN calls `zaken.CreateZaak(submitted, now)`. The submit endpoint
+never branches on `Zgw:Enabled` itself — DI already picked the implementation, so the endpoint
+just asks the seam for `(Referentie, Status)` and returns exactly that in the unchanged
+`SubmitApplicationResponse`. Under the default (local) source this returns precisely what was
+just computed; under OpenZaak, three calls happen in order:
+
+1. **POST zaak** (`{ZrcBaseUrl}/zaken`) — `zaaktype` resolved from `Zgw:ZaaktypeUrls[aanvraag.Type]`
+   (OpenZaak validates the URL by fetching it), `bronorganisatie`/`verantwoordelijkeOrganisatie`
+   (RSIN) from config, `identificatie` set to the **same** reference `ApplicationStore.Submit`
+   already generated — so the human-readable reference matches in both places, not two
+   independently-generated ones.
+2. **POST status** (`{ZrcBaseUrl}/statussen`) — `statustype` resolved via a Catalogi GET
+   (`statustypen?zaaktype=...`, lowest `volgnummer`); marks the zaak as freshly opened.
+3. **POST rol** (`{ZrcBaseUrl}/rollen`) — `roltype` resolved via a Catalogi GET
+   (`roltypen?zaaktype=...&omschrijvingGeneriek=initiator`); `betrokkeneIdentificatie.inpBsn`
+   set to the aanvraag's owner (BSN) — the current stand-in for real identity (WP-53).
+
+The created zaak's `identificatie` becomes the returned `Referentie`; its status maps to the
+same coarse `InBehandeling` shape `ZgwZaakMapper` already uses for a freshly-opened zaak
+(`ZgwZaakMapper.ToCreatedStatusDto`).
+
+ponytail shortcuts, marked at the call sites: (a) "first statustype/roltype Catalogi returns"
+rather than a fully-configured per-type map — fine while a zaaktype has exactly one initial
+status and initiator role; (b) no compensating transaction — if any ZGW call throws, the
+aanvraag is already `Submitted` locally with no matching zaak (acceptable for a demo backend;
+a production arc needs retry/reconciliation or an outbox before trusting this dual-write).
 
 ## The ZGW client (`backend/src/BigRegister.Api/Zgw/`)
 
@@ -75,7 +110,14 @@ path async if OpenZaak becomes the default.
   "ZrcBaseUrl": "https://open-zaak.example/zaken/api/v1",
   "ZtcBaseUrl": "https://open-zaak.example/catalogi/api/v1",
   "ClientId": "big-register", "Secret": "<from a secret store>",
-  "UserId": "<session user>", "UserRepresentation": "<session name>"
+  "UserId": "<session user>", "UserRepresentation": "<session name>",
+  // WP-50 (create-zaak): RSINs + the aanvraag-type → zaaktype URL map.
+  "Bronorganisatie": "<RSIN>", "VerantwoordelijkeOrganisatie": "<RSIN>",
+  "ZaaktypeUrls": {
+    "registratie": "https://open-zaak.example/catalogi/api/v1/zaaktypen/<uuid>",
+    "herregistratie": "https://open-zaak.example/catalogi/api/v1/zaaktypen/<uuid>",
+    "intake": "https://open-zaak.example/catalogi/api/v1/zaaktypen/<uuid>"
+  }
 }
 ```
 
@@ -111,9 +153,9 @@ Principles this demonstrates:
   comment in `ZgwZaakMapper` show where the ACL is deliberately thin — an ACL need not be
   complete on day one, but its shortcuts should be visible.
 
-Caveat: today only the cases **read** path has a source interface (`IZaakSource`). Other BFF
+Caveat: `IZaakSource` now covers the cases **read + create** path (WP-49/50). Other BFF
 endpoints still read `SeedData`/static stores directly — ACL-ready (the DTO seam exists) but not
-yet swappable. That is the WP-50/51/52 roadmap, plus the two cross-cutting WPs the arc needs for
+yet swappable. That is the WP-51/52 roadmap, plus the two cross-cutting WPs the arc needs for
 production: **WP-53** (a real per-request identity seam + citizen-scoping — today the owner/BSN
 is stubbed) and **WP-54** (a docker OpenZaak harness + opt-in integration test — today everything
 is fixture/mock-tested against no live instance).