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/037_class_required_form.sql

@@ -0,0 +1,25 @@
+-- =============================================================================
+-- 037: A class can optionally require a form
+-- =============================================================================
+-- A class may point at one form in the same space (a waiver or intake form).
+-- When set, signing up for any session of that class requires the member to
+-- have a completed submission of that form on file (the "waiver on file"
+-- model). The gate is enforced in the application
+-- (lib/classes-logic.ts + lib/actions/classes.ts), mirroring the existing
+-- equipment required-certification gate: a classes.manage holder can override
+-- and sign a member up on their behalf. The submission check reads
+-- form_submissions via the service client (boolean only) since a class
+-- manager need not hold forms.manage.
+--
+-- ON DELETE SET NULL: deleting the form just drops the requirement; existing
+-- signups are unaffected.
+--
+-- No RLS change: classes already has additive default-deny policies and this
+-- is an additional nullable column on it.
+--
+-- Idempotent: ADD COLUMN IF NOT EXISTS.
+-- =============================================================================
+
+ALTER TABLE public.classes
+  ADD COLUMN IF NOT EXISTS required_form_id uuid
+  REFERENCES public.forms(id) ON DELETE SET NULL;

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()