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/Acceptance/IntakeSubmissionTests.cs

@@ -8,13 +8,13 @@ using BigRegister.Tests.Builders;
 namespace BigRegister.Tests.Acceptance;
 
 /// <summary>
-/// Behaviour-level tests for the scholing-threshold enforcement (WP-69) over both live HTTP
-/// paths — <c>POST /applications/{id}/submit</c> (the wizard's real path) and the legacy
-/// <c>POST /intakes</c> (dead from the UI, still a live crafted-POST surface). Built through
-/// the <see cref="Given"/> type-state builder, mirroring <see cref="BesluitLifecycleTests"/>
-/// rather than the full wizard/upload dance — the builder's default owner IS
-/// <see cref="BigRegister.Api.Domain.Authorization.StubIdentityProvider"/>'s default caller,
-/// so no header juggling.
+/// Behaviour-level tests for the scholing-threshold enforcement (WP-69) over
+/// <c>POST /applications/{id}/submit</c> (the wizard's real path — WP-72 deleted the legacy
+/// <c>POST /intakes</c> endpoint this once also covered). Built through the <see
+/// cref="Given"/> type-state builder, mirroring <see cref="BesluitLifecycleTests"/> rather
+/// than the full wizard/upload dance — the builder's default owner IS <see
+/// cref="BigRegister.Api.Domain.Authorization.StubIdentityProvider"/>'s default caller, so
+/// no header juggling.
 /// </summary>
 public class IntakeSubmissionTests(TestWebApplicationFactory factory) : IClassFixture<TestWebApplicationFactory>
 {
@@ -23,7 +23,7 @@ public class IntakeSubmissionTests(TestWebApplicationFactory factory) : IClassFi
   private static void Persist(Aanvraag aanvraag)
   {
     using var db = Db.Create();
-    db.Applications.Add(aanvraag);
+    db.Applications.Add(aanvraag.ToEntity());
     db.SaveChanges();
   }
 
@@ -44,8 +44,7 @@ public class IntakeSubmissionTests(TestWebApplicationFactory factory) : IClassFi
     Assert.Equal(HttpStatusCode.BadRequest, res.StatusCode);
 
     // ...and the aanvraag is left a retryable Concept, never marked Submitted.
-    var stillConcept = ApplicationStore.GetAny(aanvraag.Id)!;
-    Assert.False(stillConcept.Submitted);
+    Assert.IsType<Aanvraag.Concept>(ApplicationStore.GetAny(aanvraag.Id));
   }
 
   [Fact]
@@ -121,16 +120,4 @@ public class IntakeSubmissionTests(TestWebApplicationFactory factory) : IClassFi
     var body = (await res.Content.ReadFromJsonAsync<SubmitApplicationResponse>())!;
     Assert.Equal("Afgewezen", body.Status.Tag);
   }
-
-  [Fact]
-  public async Task Legacy_intakes_endpoint_enforces_it_too()
-  {
-    // Given no aanvraag needed — the legacy endpoint mints its own reference.
-    // When a crafted POST hits the dead-from-the-UI /intakes endpoint below threshold,
-    // with no scholing answer...
-    var res = await _client.PostAsJsonAsync("/api/v1/intakes", new { uren = 500 });
-
-    // Then it is rejected too — the crafted-POST surface this WP closes.
-    Assert.Equal(HttpStatusCode.BadRequest, res.StatusCode);
-  }
 }