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/Dtos.cs

@@ -71,3 +71,39 @@ public sealed record HerregistratieRequest(int Uren, IReadOnlyList<DocumentRefDt
 public sealed record ChangeRequestRequest(string Straat, string Postcode, string Woonplaats);
 
 public sealed record ReferentieResponse(string Referentie);
+
+// --- Applications (aanvragen): the system of record for the dashboard. ---
+
+// Status is a discriminated union by Tag (Concept | InBehandeling | Goedgekeurd |
+// Afgewezen); only the fields relevant to a tag are populated. Mirrors the FE union.
+public sealed record AanvraagStatusDto(
+    string Tag,
+    int? StepIndex = null,
+    int? StepCount = null,
+    string? Referentie = null,
+    bool? Manual = null,
+    string? Reden = null);
+
+public sealed record ApplicationSummaryDto(
+    string Id, string Type, AanvraagStatusDto Status,
+    IReadOnlyList<string> DocumentIds,
+    string CreatedAt, string UpdatedAt, string? SubmittedAt);
+
+public sealed record ApplicationDetailDto(
+    string Id, string Type, AanvraagStatusDto Status,
+    System.Text.Json.JsonElement? Draft,
+    IReadOnlyList<string> DocumentIds,
+    string CreatedAt, string UpdatedAt, string? SubmittedAt);
+
+public sealed record CreateApplicationRequest(string Type);
+
+public sealed record DraftSyncRequest(
+    System.Text.Json.JsonElement Draft, int StepIndex, int StepCount,
+    IReadOnlyList<string>? DocumentIds = null);
+
+// Submit carries only the fields the server re-validates per wizard type.
+public sealed record SubmitApplicationRequest(
+    string? DiplomaHerkomst = null, int? Uren = null,
+    IReadOnlyList<DocumentRefDto>? Documents = null);
+
+public sealed record SubmitApplicationResponse(string Referentie, AanvraagStatusDto Status);