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>

backend/src/BigRegister.Api/Program.cs

@@ -244,10 +244,15 @@ api.MapGet("/applications/{id}", (string id) =>
 
 api.MapPost("/applications", (CreateApplicationRequest req) =>
 {
-  var a = ApplicationStore.Create(req.Type, DocumentStore.DemoOwner);
+  var a = ApplicationStore.CreateConcept(req.Type, DocumentStore.DemoOwner);
+  if (a is null)
+    return Results.Problem(
+        detail: "U hebt al een concept van dit type. Rond dat eerst af of verwijder het.",
+        statusCode: StatusCodes.Status409Conflict);
   return Results.Created($"/api/v1/applications/{a.Id}", a.ToDetailDto(DateTimeOffset.UtcNow));
 })
-.Produces<ApplicationDetailDto>(StatusCodes.Status201Created);
+.Produces<ApplicationDetailDto>(StatusCodes.Status201Created)
+.ProducesProblem(StatusCodes.Status409Conflict);
 
 // Draft sync per step — idempotent; keep it debounced on the client (it is chatty).
 api.MapPut("/applications/{id}", (string id, DraftSyncRequest req) =>