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>