feat(registratie): WP-35 — one Concept per case type (server-enforced)

Make "at most one unsubmitted Concept per type" a server invariant instead of a
client-only convenience. ApplicationStore.Create → CreateConcept guards atomically
under the write gate and POST /applications returns 409 when a duplicate would be
created. The FE draft-sync recovers from the 409 by adopting the existing Concept
(ensureId → findConcept) rather than erroring — one-per-type means the second
attempt lands on the existing draft. Typed client regenerated (documents the 409).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/registratie/application/draft-sync.spec.ts

@@ -95,6 +95,27 @@ describe('createDraftSync', () => {
       const r = await draftSync.submit({});
       expect(r.ok).toBe(false);
     });
+
+    it('recovers from a create conflict by adopting the existing Concept (WP-35)', async () => {
+      // Server enforces one Concept per type: a stale/cross-tab create is rejected (409),
+      // and ensureId adopts the existing Concept from the list instead of erroring.
+      const create = vi.fn().mockRejectedValue({ status: 409 });
+      const list = vi.fn().mockResolvedValue([
+        {
+          id: 'existing-1',
+          type: 'registratie',
+          status: { tag: 'Concept', stepIndex: 1, stepCount: 3 },
+          createdAt: '2026-07-23T10:00:00Z',
+          updatedAt: '2026-07-23T10:00:00Z',
+        },
+      ]);
+      const submit = vi.fn().mockResolvedValue({ id: 'existing-1', autoApprovable: true });
+      const { draftSync } = setup({ create, list, submit });
+
+      const r = await draftSync.submit({});
+      expect(r.ok).toBe(true);
+      expect(submit).toHaveBeenCalledWith('existing-1', {}); // adopted, not a new id
+    });
   });
 
   describe('flushPending (CanDeactivate guard / beforeunload)', () => {

src/app/registratie/application/draft-sync.ts

@@ -63,17 +63,28 @@ export function createDraftSync(deps: DraftSyncDeps) {
   const ensureId = async (): Promise<string> => {
     await resumeGate;
     if (id) return id;
-    ensuring ??= adapter.create(deps.type).then((newId) => {
-      id = newId;
-      // Stamp the id into the URL (no navigation) so a reload resumes this Concept.
-      void router!.navigate([], {
-        relativeTo: route!,
-        queryParams: { aanvraag: newId },
-        queryParamsHandling: 'merge',
-        replaceUrl: true,
+    ensuring ??= adapter
+      .create(deps.type)
+      // WP-35: one Concept per type is server-enforced. Within a tab the resumeGate
+      // already prevents a second create, but a cross-tab/stale race can still hit the
+      // server's guard (409) — recover by adopting the existing Concept instead of
+      // erroring. Only recover when one actually exists; otherwise surface the failure.
+      .catch(async (e) => {
+        const existing = await findConcept();
+        if (existing) return existing;
+        throw e;
+      })
+      .then((newId) => {
+        id = newId;
+        // Stamp the id into the URL (no navigation) so a reload resumes this Concept.
+        void router!.navigate([], {
+          relativeTo: route!,
+          queryParams: { aanvraag: newId },
+          queryParamsHandling: 'merge',
+          replaceUrl: true,
+        });
+        return newId;
       });
-      return newId;
-    });
     return ensuring;
   };
 

src/app/shared/infrastructure/api-client.ts

@@ -832,6 +832,12 @@ export class ApiClient {
             result201 = _responseText === "" ? null : JSON.parse(_responseText, this.jsonParseReviver) as ApplicationDetailDto;
             return result201;
             });
+        } else if (status === 409) {
+            return response.text().then((_responseText) => {
+            let result409: any = null;
+            result409 = _responseText === "" ? null : JSON.parse(_responseText, this.jsonParseReviver) as ProblemDetails;
+            return throwException("Conflict", status, _responseText, _headers, result409);
+            });
         } else if (status !== 200 && status !== 204) {
             return response.text().then((_responseText) => {
             return throwException("An unexpected server error occurred.", status, _responseText, _headers);