certifications.ts: create/update/delete/list cert types (gated by
certifications.manage; delete refused once granted, archive instead),
and grant/revoke/renew/listMemberCertifications/getMyCertifications
(gated by certifications.grant, the Instructor capability). Grant
verifies cert and target member are in-space and the cert is active,
computes expiry from validity_months unless overridden, and relies on
the partial unique index for one active grant per member+cert. Revoke
is soft; nothing is ever deleted. getMyCertifications takes no params
(member resolved from session; RLS restricts to own rows).
certifications-guard.ts mirrors the forms guard for the admin pages.
Barrel re-export and API_REFERENCE updated.
