Upload feature (a): BFF endpoints + category config + tests

- Domain/Documents: server-owned category config per wizard + authoritative
  type/size validation (DocumentRules).
- Data/DocumentStore: in-memory metadata store (no file bytes/PII) + audit log;
  user delete (owner-scoped, 409 once linked), admin delete (role seam via
  X-Admin header), link-on-submit, poll-by-localId status.
- Program.cs: GET /uploads/categories, POST /uploads (multipart, excluded from
  OpenAPI — hand-written on FE), GET /uploads/status, DELETE /uploads/{id},
  DELETE /admin/uploads/{id}. Submit links digital docs + records post-delivery.
- Contracts extended (DocumentRefDto on registratie/herregistratie submit);
  regenerated NSwag client + swagger.json (drift check stays green).
- Tests: 16 new (endpoints + DocumentRules); dotnet test 44/44.

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

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

@@ -46,11 +46,28 @@ public sealed record DuoLookupDto(IReadOnlyList<DuoDiplomaDto> Diplomas, ManualD
 
 public sealed record IntakePolicyDto(int ScholingThreshold);
 
+// --- Document upload contracts ---
+
+public sealed record DocumentCategoryDto(
+    string CategoryId, string Label, string Description, bool Required,
+    IReadOnlyList<string> AcceptedTypes, int MaxSizeMb, bool Multiple, bool AllowPostDelivery);
+
+public sealed record UploadCategoriesDto(IReadOnlyList<DocumentCategoryDto> Categories);
+
+public sealed record UploadResponse(string DocumentId, string LocalId);
+
+public sealed record UploadStatusItemDto(string LocalId, string Status, string? DocumentId);
+public sealed record UploadStatusDto(IReadOnlyList<UploadStatusItemDto> Results);
+
+// Per-category delivery intent on submit: digital categories carry their DocumentId,
+// post-delivery categories carry Channel="post".
+public sealed record DocumentRefDto(string CategoryId, string Channel, string? DocumentId = null);
+
 // Submit requests carry only the fields the server re-validates (UX-only fields
 // stay on the client). ponytail: a real submit would carry the full application.
-public sealed record RegistratieRequest(string DiplomaHerkomst);
+public sealed record RegistratieRequest(string DiplomaHerkomst, IReadOnlyList<DocumentRefDto>? Documents = null);
 public sealed record IntakeRequest(int Uren);
-public sealed record HerregistratieRequest(int Uren);
+public sealed record HerregistratieRequest(int Uren, IReadOnlyList<DocumentRefDto>? Documents = null);
 public sealed record ChangeRequestRequest(string Straat, string Postcode, string Woonplaats);
 
 public sealed record ReferentieResponse(string Referentie);