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/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);