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/tests/BigRegister.Tests/ApplicationTests.cs

@@ -12,6 +12,10 @@ public class ApplicationTests(TestWebApplicationFactory factory) : IClassFixture
 
   private async Task<ApplicationDetailDto> Create(string type = "registratie")
   {
+    // WP-35: one Concept per type is now server-enforced, and these tests share one DB
+    // (IClassFixture). Clear any leftover Concept so each test starts from a clean slate.
+    foreach (var s in (await List())!.Where(x => x.Status.Tag == "Concept"))
+      await _client.DeleteAsync($"/api/v1/applications/{s.Id}");
     var res = await _client.PostAsJsonAsync("/api/v1/applications", new { type });
     Assert.Equal(HttpStatusCode.Created, res.StatusCode);
     return (await res.Content.ReadFromJsonAsync<ApplicationDetailDto>())!;
@@ -90,6 +94,33 @@ public class ApplicationTests(TestWebApplicationFactory factory) : IClassFixture
     Assert.Equal(HttpStatusCode.Conflict, again.StatusCode);
   }
 
+  // --- WP-35: one Concept per case type (server-enforced) ---
+
+  [Fact]
+  public async Task Creating_a_second_concept_of_the_same_type_conflicts()
+  {
+    await Create("herregistratie");
+    var dup = await _client.PostAsJsonAsync("/api/v1/applications", new { type = "herregistratie" });
+    Assert.Equal(HttpStatusCode.Conflict, dup.StatusCode);
+  }
+
+  [Fact]
+  public async Task A_concept_of_a_different_type_is_allowed()
+  {
+    await Create("registratie");
+    var other = await _client.PostAsJsonAsync("/api/v1/applications", new { type = "herregistratie" });
+    Assert.Equal(HttpStatusCode.Created, other.StatusCode);
+  }
+
+  [Fact]
+  public async Task A_new_concept_is_allowed_once_the_previous_one_is_submitted()
+  {
+    var a = await Create("registratie");
+    (await _client.PostAsJsonAsync($"/api/v1/applications/{a.Id}/submit", new { diplomaHerkomst = "duo" })).EnsureSuccessStatusCode();
+    var next = await _client.PostAsJsonAsync("/api/v1/applications", new { type = "registratie" });
+    Assert.Equal(HttpStatusCode.Created, next.StatusCode);
+  }
+
   [Fact]
   public async Task Cancel_concept_removes_it()
   {