The members branch was inserting space_members directly from the
browser with user_id: crypto.randomUUID() -- a fabricated id that
violates the auth.users FK (offline members must be user_id NULL) and
bypassed all server-side validation. Now maps columns and calls the
existing admin-gated, Zod-validated, batched importMembers action;
surfaces count + skipped. Dropped the now-unused spaceId prop.