feat(bff): documents endpoint accepts the base64 file and forwards it to the domain (refs #103)

The self-service documents endpoint takes { contentBase64, fileName, contentType }
as JSON (bsn from the token) and forwards it via IDomainClient.ProvideDocumentsAsync.
Regenerates openapi.json + the Angular client (postSelfServiceRegistrationsIdDocuments
now takes a ProvideDocumentsRequest body).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/bff/Bff.Api/Program.cs

@@ -109,13 +109,15 @@ app.MapPost("/self-service/registrations/{id}/withdraw", async (string id, Claim
 // forwarded to the domain, which owner-scopes the action and completes the WachtOpDocumenten task; a
 // registration that is unknown or not the caller's comes back 404. The real file upload + ZGW storage
 // is S-10b — this is the trigger that unblocks the process.
-app.MapPost("/self-service/registrations/{id}/documents", async (string id, ClaimsPrincipal user, IDomainClient domain, CancellationToken ct) =>
+app.MapPost("/self-service/registrations/{id}/documents", async (string id, ProvideDocumentsRequest body, ClaimsPrincipal user, IDomainClient domain, CancellationToken ct) =>
 {
     var bsn = user.FindFirstValue("bsn");
     if (string.IsNullOrWhiteSpace(bsn))
         return Results.BadRequest("The token carries no bsn claim.");
+    if (string.IsNullOrWhiteSpace(body?.ContentBase64))
+        return Results.BadRequest("A document is required.");
 
-    var provided = await domain.ProvideDocumentsAsync(id, bsn, ct);
+    var provided = await domain.ProvideDocumentsAsync(id, bsn, body.ContentBase64, body.FileName, body.ContentType, ct);
     return provided ? Results.NoContent() : Results.NotFound();
 })
     .RequireAuthorization()
@@ -163,6 +165,10 @@ app.Run();
 /// <summary>The behandelaar's decision on a registration.</summary>
 public sealed record DecideRequest(string Besluit);
 
+/// <summary>A diploma upload from the self-service portal — the file base64-encoded client-side, with
+/// its name and MIME type. The bsn is taken from the DigiD token, not this body.</summary>
+public sealed record ProvideDocumentsRequest(string ContentBase64, string? FileName = null, string? ContentType = null);
+
 // Behandel (medewerker-realm) authentication + authorization wiring (ADR-0013).
 internal static class BehandelAuth
 {