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.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;
   };