Splits backend CallerIdentity into the two ADR-0002 §3 actor kinds
(ZorgverlenerCaller/MedewerkerCaller), a stub X-Medewerker/X-Rollen header
path mirroring WP-53's citizen stub, and Authz.CanBeoordelen as the first
medewerker capability — backend-only, no consumer until WP-64. Also fixes
the backlog README's stale WP-61 status (done, but table said todo).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>