Mijn aanvragen (A): backend Aanvraag store + lifecycle endpoints

Adds the backend-owned Aanvraag aggregate (PRD 0001, phase A) — the system of
record the dashboard will read. In-memory static store mirroring DocumentStore.

- ApplicationStore: create/get/list/draft-sync/cancel/submit; status COMPUTED ON
  READ (Mappers.ToStatusDto(now)) so auto-approval is pure timestamp arithmetic,
  no timers/jobs (ProcessingWindow = 8s).
- Endpoints: GET /applications, GET/POST/PUT/DELETE /applications/{id},
  POST /applications/{id}/submit.
- Lifecycle: registratie duo -> auto (Goedgekeurd after window), handmatig ->
  manual pending (no 422); herregistratie/intake 0 uren -> Afgewezen else auto.
  Cancel blocks submitted aanvragen (409, no withdrawal in scope).
- Old /registrations endpoint + RejectRegistratie 422 left intact (retire in E).
- ApplicationTests: lifecycle + auto-approve window boundary (pure). 54/54 green.

Also checks in the PRD (docs/prd/0001).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/BigRegister.Api/Contracts/Mappers.cs

@@ -1,3 +1,4 @@
+using BigRegister.Api.Data;
 using BigRegister.Domain.Diplomas;
 using BigRegister.Domain.Documents;
 using BigRegister.Domain.People;
@@ -34,4 +35,27 @@ public static class Mappers
 
     public static DocumentCategoryDto ToDto(this DocumentCategory c) => new(
         c.CategoryId, c.Label, c.Description, c.Required, c.AcceptedTypes, c.MaxSizeMb, c.Multiple, c.AllowPostDelivery);
+
+    // Aanvraag status is COMPUTED ON READ: an auto-approvable submission reports
+    // Goedgekeurd once past the processing window, else In behandeling; a manual case
+    // stays In behandeling forever (awaits the unbuilt backoffice). Pure — testable
+    // by passing different `now` values without waiting for the wall clock.
+    public static AanvraagStatusDto ToStatusDto(this Aanvraag a, DateTimeOffset now)
+    {
+        if (!a.Submitted)
+            return new("Concept", StepIndex: a.StepIndex, StepCount: a.StepCount);
+        if (a.Reden is not null)
+            return new("Afgewezen", Referentie: a.Referentie, Reden: a.Reden);
+        if (a.AutoApprovable && now > a.SubmittedAt!.Value + ApplicationStore.ProcessingWindow)
+            return new("Goedgekeurd", Referentie: a.Referentie);
+        return new("InBehandeling", Referentie: a.Referentie, Manual: !a.AutoApprovable);
+    }
+
+    public static ApplicationSummaryDto ToSummaryDto(this Aanvraag a, DateTimeOffset now) => new(
+        a.Id, a.Type, a.ToStatusDto(now), a.DocumentIds,
+        a.CreatedAt.ToString("o"), a.UpdatedAt.ToString("o"), a.SubmittedAt?.ToString("o"));
+
+    public static ApplicationDetailDto ToDetailDto(this Aanvraag a, DateTimeOffset now) => new(
+        a.Id, a.Type, a.ToStatusDto(now), a.Draft, a.DocumentIds,
+        a.CreatedAt.ToString("o"), a.UpdatedAt.ToString("o"), a.SubmittedAt?.ToString("o"));
 }