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/src/BigRegister.Api/Program.cs

@@ -189,24 +189,6 @@ api.MapPost("/registrations", (RegistratieRequest req, HttpContext ctx) =>
 .Produces<ReferentieResponse>()
 .ProducesProblem(StatusCodes.Status422UnprocessableEntity);
 
-api.MapPost("/herregistraties", (HerregistratieRequest req, HttpContext ctx) =>
-    Submit(ctx, "herregistratie", SubmissionRules.RejectZeroUren(req.Uren), req.Documents))
-.Produces<ReferentieResponse>()
-.ProducesProblem(StatusCodes.Status422UnprocessableEntity);
-
-api.MapPost("/intakes", (IntakeRequest req, HttpContext ctx) =>
-{
-  // WP-69: completeness check outside Submit(...) — deliberately not folded into `reject`,
-  // so this 400 is never cached in IdempotencyStore the way a 422 rejection would be.
-  var reject = SubmissionRules.RejectZeroUren(req.Uren);
-  if (reject is null && IntakePolicy.RejectIncompleteScholing(req.Uren, req.AanvullendeScholing, req.ScholingPunten) is { } incomplete)
-    return Results.Problem(detail: incomplete, statusCode: StatusCodes.Status400BadRequest);
-  return Submit(ctx, "intake", reject);
-})
-.Produces<ReferentieResponse>()
-.ProducesProblem(StatusCodes.Status400BadRequest)
-.ProducesProblem(StatusCodes.Status422UnprocessableEntity);
-
 api.MapPost("/change-requests", (ChangeRequestRequest req, HttpContext ctx) =>
     Submit(ctx, "telefoonwijziging", SubmissionRules.RejectPhoneChange(req.Telefoon)))
 .Produces<ReferentieResponse>()
@@ -344,7 +326,7 @@ api.MapDelete("/applications/{id}", (string id, HttpContext ctx) =>
 {
   var a = ApplicationStore.Get(id, ctx.Zorgverlener().Bsn);
   if (a is null) return Results.NotFound();
-  if (a.Submitted)
+  if (a is not Aanvraag.Concept)
     return Results.Problem(detail: "Een ingediende aanvraag kan niet worden geannuleerd.", statusCode: StatusCodes.Status409Conflict);
   ApplicationStore.Delete(id, ctx.Zorgverlener().Bsn);
   return Results.NoContent();
@@ -359,7 +341,7 @@ api.MapPost("/applications/{id}/submit", (string id, SubmitApplicationRequest re
 {
   var existing = ApplicationStore.Get(id, ctx.Zorgverlener().Bsn);
   if (existing is null) return Results.NotFound();
-  if (existing.Submitted)
+  if (existing is not Aanvraag.Concept)
     return Results.Problem(detail: "Aanvraag is al ingediend.", statusCode: StatusCodes.Status409Conflict);
 
   // Per wizard type: what rejects the submission (→ Afgewezen) and whether it auto-approves.
@@ -404,7 +386,7 @@ api.MapPost("/applications/{id}/submit", (string id, SubmitApplicationRequest re
   // failure (an orphan zaak from a rolled-back-then-retried submit is worse than a flagged
   // one, see openzaak-integration.md's "Write resilience" section). Each ZGW half is caught
   // separately so a create-zaak failure doesn't also skip the (still-local) document link.
-  var referentie = submitted.Referentie!;
+  var referentie = submitted.Referentie;
   var status = submitted.ToStatusDto(DateTimeOffset.UtcNow);
   string? zaakUrl = null;
   try
@@ -525,7 +507,8 @@ api.MapPost("/beoordeling/{id}/besluit", (string id, RecordBesluitRequest req, H
       }
       catch (Exception ex)
       {
-        RecordZgwDivergence(ctx, a.Id, updated!.Referentie ?? a.Id, ex);
+        // WP-73: Aanvraag.Decided's Referentie is required/non-null — no `?? a.Id` fallback needed.
+        RecordZgwDivergence(ctx, a.Id, updated!.Referentie, ex);
       }
 
       return Results.Ok(new RecordBesluitResponse(updated!.ToStatusDto(now)));