feat(domain,bff): current-registration lookup for self-service resume (refs #111)

Backend half of S-26. The domain gains IRegistrationStore.FindOpenByBsnAsync (the
citizen's non-terminal INGEDIEND/IN_BEHANDELING registration) + GET
/registrations/current?bsn=; the BFF adds owner-scoped GET /self-service/registrations
(bsn from the DigiD token) returning that registration or 204 when none. Regenerated
services/bff/openapi.json for the new endpoint (the api-client is generated from it).
Store + BFF endpoint unit tests; acceptance/BFF fakes updated for the new members.

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

services/bff/Bff.Tests/BffFactory.cs

@@ -82,6 +82,18 @@ internal sealed class FakeDomainClient : IDomainClient
         return Task.FromResult(Result);
     }
 
+    public string? CurrentQueriedBsn { get; private set; }
+
+    /// <summary>The current open registration the fake domain returns (null → the citizen has none in
+    /// flight, so the BFF replies 204). Tests set this to exercise resume.</summary>
+    public CurrentRegistration? Current { get; set; }
+
+    public Task<CurrentRegistration?> GetCurrentRegistrationAsync(string bsn, CancellationToken ct = default)
+    {
+        CurrentQueriedBsn = bsn;
+        return Task.FromResult(Current);
+    }
+
     public (string RegistrationId, string Bsn)? Withdrawn { get; private set; }
 
     /// <summary>Whether the fake domain reports the withdrawal as done (true → 204) or not-found/not-owned