Migration 030 adds certifications (per-space cert types, optional
validity_months, is_active archive) and member_certifications (per-member
grants; expires_at snapshotted at grant time; soft revoke; partial unique
on one active grant per member+cert). RLS is additive and default-deny:
cert types readable by any space member, write-gated by
certifications.manage; grants readable by managers/granters (all) or the
member (own), written by certifications.grant, with no DELETE policy so
the history is immutable. No anonymous path.
Two new permission codes in the catalog (group Certifications):
certifications.manage and certifications.grant (the Instructor
capability, assignable to any role via the existing additive model).
Seeded to board and backfilled. Mirrored idempotently in schema.sql;
types and schema docs updated.
