test(bff): self-service documents endpoint forwards id + bsn to the domain (refs #102)

RED: POST /self-service/registrations/{id}/documents requires a digid token, takes
the bsn from the token, forwards to the domain, and relays the domain's 404 for an
unknown/not-owned registration. Adds the IDomainClient.ProvideDocumentsAsync port +
client + fake; the endpoint itself follows.

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

services/bff/Bff.Api/DownstreamClients.cs

@@ -27,6 +27,11 @@ public interface IDomainClient
     /// unknown or not the caller's (404), so the BFF can relay a 404 rather than a 500.</summary>
     Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default);
 
+    /// <summary>Provide the documents the caller's own registration is waiting for ("documenten
+    /// aanleveren"). Owner-scoped by <paramref name="bsn"/>. Returns <c>false</c> when the domain
+    /// reports the registration is unknown or not the caller's (404), so the BFF can relay a 404.</summary>
+    Task<bool> ProvideDocumentsAsync(string registrationId, string bsn, CancellationToken ct = default);
+
     /// <summary>The behandelaar's werkbak — registrations awaiting beoordeling.</summary>
     Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default);
 
@@ -63,6 +68,17 @@ public sealed class DomainClient(HttpClient http) : IDomainClient
         return true;
     }
 
+    public async Task<bool> ProvideDocumentsAsync(string registrationId, string bsn, CancellationToken ct = default)
+    {
+        using var response = await http.PostAsJsonAsync(
+            $"registrations/{registrationId}/documents", new { bsn }, ct);
+        // The domain 404s an unknown or not-owned registration; relay that rather than fail hard.
+        if (response.StatusCode == System.Net.HttpStatusCode.NotFound)
+            return false;
+        response.EnsureSuccessStatusCode();
+        return true;
+    }
+
     public async Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
         => await http.GetFromJsonAsync<List<WerkbakItem>>("behandel/werkbak", ct) ?? [];