refactor(backend): delete dead legacy endpoints, make domain types unions (WP-72 + WP-73)

Two work packages in one commit because both edit Program.cs and splitting
them would leave a commit that does not build.

WP-72 — deletes POST /api/v1/intakes and /herregistraties. Both were dead
from the UI (the wizard submits via /applications/{id}/submit) and strictly
less capable: they minted a bare reference and wrote no Aanvraag, made no
ZGW call, and did no document-ownership check. The shared Submit(...) helper
survives — /registrations and /change-requests still use it. WP-69 hardened
/intakes with a 400 last session; removing the surface is the stronger fix,
and WP-69's /applications/{id}/submit enforcement is untouched.

WP-73 — RegistrationStatus becomes an abstract record with three sealed
variants behind a private base ctor, so only Geregistreerd carries a
herregistratie deadline and reden is required on Geschorst/Doorgehaald
(matching the FE union, which was already right). HerregistratieRule
.IsStatusConsistent and its test are deleted: the type now guarantees what
the runtime check was for, and the test could no longer construct the
illegal state it existed to catch.

Aanvraag splits into a Concept | Submitted | Decided union with the EF row
demoted to AanvraagEntity behind a two-way mapper. Submitted carries a
non-null Referentie and SubmittedAt, and Decided.Afgewezen/MeerInfoGevraagd
require a Toelichting — so the five Referentie! null-forgiving derefs in
StatusAt are gone, not merely suppressed. IZaakSource.CreateZaak narrows to
Aanvraag.Submitted, removing the same class of deref in both zaak sources.

Draft is now cleared on submit rather than lingering: ApplicationStore's
doc-comment claimed "Concept only" but Submit never cleared it. Verified
nothing reads a submitted aanvraag's draft (draft-sync's applyResume only
resumes unsubmitted wizards), so the comment is now true instead of
aspirational.

No migration, no schema change, no wire change — RegistrationStatusDto and
the application DTOs are byte-identical, confirmed against a live swagger.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/ApplicationTests.cs

@@ -3,6 +3,7 @@ using System.Net.Http.Json;
 using BigRegister.Api.Contracts;
 using BigRegister.Api.Data;
 using BigRegister.Domain.Applications;
+using BigRegister.Tests.Builders;
 using Microsoft.AspNetCore.Mvc.Testing;
 
 namespace BigRegister.Tests;
@@ -227,25 +228,14 @@ public class ApplicationTests(TestWebApplicationFactory factory) : IClassFixture
 
   // --- Auto-approval is computed on read: exercise the window boundary without waiting. ---
 
-  private static Aanvraag Accepted(bool autoApprovable) => new()
-  {
-    Id = "x",
-    Type = "registratie",
-    Owner = "test",
-    Submitted = true,
-    AutoApprovable = autoApprovable,
-    Referentie = "BIG-2026-1",
-    SubmittedAt = DateTimeOffset.UtcNow,
-    CreatedAt = DateTimeOffset.UtcNow,
-    UpdatedAt = DateTimeOffset.UtcNow,
-  };
+  private static Aanvraag.Submitted Accepted(bool autoApprovable) =>
+      Given.Concept(type: "registratie", owner: "test").Submitted(autoApprovable).Build();
 
   [Fact]
   public void AutoApprovable_flips_to_goedgekeurd_after_the_window()
   {
     var a = Accepted(autoApprovable: true);
-    Assert.NotNull(a.SubmittedAt);
-    var t0 = a.SubmittedAt.Value;
+    var t0 = a.SubmittedAt;
     Assert.Equal("InBehandeling", a.ToStatusDto(t0 + ApplicationStore.ProcessingWindow - TimeSpan.FromSeconds(1)).Tag);
     Assert.Equal("Goedgekeurd", a.ToStatusDto(t0 + ApplicationStore.ProcessingWindow + TimeSpan.FromSeconds(1)).Tag);
   }
@@ -254,8 +244,7 @@ public class ApplicationTests(TestWebApplicationFactory factory) : IClassFixture
   public void Manual_case_never_auto_advances()
   {
     var a = Accepted(autoApprovable: false);
-    Assert.NotNull(a.SubmittedAt);
-    var far = a.SubmittedAt.Value + ApplicationStore.ProcessingWindow + TimeSpan.FromDays(1);
+    var far = a.SubmittedAt + ApplicationStore.ProcessingWindow + TimeSpan.FromDays(1);
     var status = a.ToStatusDto(far);
     Assert.Equal("InBehandeling", status.Tag);
     Assert.True(status.Manual);