Migration 032 adds classes (offering; optional payment_link and
grants_certification_id, is_active archive), class_sessions (scheduled
occurrence; denormalized space_id) and class_signups (member signup;
partial unique = one non-cancelled signup per member+session). RLS is
additive and default-deny: classes readable by managers (all) or members
(active only), class_sessions by any space member, class_signups by
managers/instructors (all) or the member (own) with instructor-only
UPDATE and no INSERT/DELETE policy so signup/cancel funnels through a
validated service-client action (capacity/waitlist/dedupe). New
classes.manage + classes.instruct permissions (group Classes) seeded to
board and backfilled. Mirrored idempotently in schema.sql; types and
schema docs updated.
