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/domain/Big.Application/Ports.cs

@@ -102,6 +102,11 @@ public interface IRegistrationStore
 
     /// <summary>Load a registration by id, or <c>null</c> if none exists.</summary>
     Task<Registration?> GetAsync(RegistrationId id, CancellationToken ct = default);
+
+    /// <summary>The citizen's current <em>open</em> (non-terminal: INGEDIEND/IN_BEHANDELING)
+    /// registration, or <c>null</c> if they have none in flight. Lets the self-service portal resume
+    /// an existing registration after a refresh (S-26); terminal registrations are not resumed.</summary>
+    Task<Registration?> FindOpenByBsnAsync(string bsn, CancellationToken ct = default);
 }
 
 /// <summary>