feat(classes): migration 037 classes.required_form_id (optional form gate)

Nullable FK classes.required_form_id -> forms(id) ON DELETE SET NULL.
Mirrored in schema.sql, types, DATABASE_SCHEMA, DB_SCHEMA_MAP. Additive
nullable column on the already-policied classes table (no RLS change);
the signup gate is enforced in the app, mirroring the equipment cert-gate.

scripts/schema.sql

@@ -2115,6 +2115,7 @@ CREATE TABLE IF NOT EXISTS public.classes (
   capacity                integer     CHECK (capacity IS NULL OR capacity > 0),
   is_active               boolean     NOT NULL DEFAULT true,
   grants_certification_id uuid        REFERENCES public.certifications(id) ON DELETE SET NULL,
+  required_form_id        uuid        REFERENCES public.forms(id) ON DELETE SET NULL,
   created_by              uuid        REFERENCES public.space_members(id) ON DELETE SET NULL,
   created_at              timestamptz NOT NULL DEFAULT now(),
   updated_at              timestamptz NOT NULL DEFAULT now()