fix(classes): atomic signup/cancel under per-session advisory lock (D11)

Deferred deeper-audit P1. signUpForClass (count->decide->insert) and
cancelMySignup (cancel->recount->promote) were non-atomic: concurrent
signups at the capacity boundary over-enrolled and concurrent cancels
double-promoted waitlisters (dynamic capacity can't be a DB constraint).
Migration 045 adds class_signup_tx/class_cancel_tx (SECURITY DEFINER,
pg_advisory_xact_lock per session) doing the decision+write atomically;
actions call the RPCs and keep all pre-checks. Removed the now-unused
effectiveCapacity/computeSignupStatus/pickPromotion imports +
isUniqueViolation (pure rules still live + tested in classes-logic).
528 tests; build clean.

scripts/schema.sql

@@ -2243,6 +2243,79 @@ INSERT INTO public.space_role_permissions (space_id, subject, permission)
 SELECT id, 'board', 'classes.instruct' FROM public.spaces
 ON CONFLICT (space_id, subject, permission) DO NOTHING;
 
+-- Atomic signup/cancel (equivalent to scripts/045). Per-session advisory
+-- xact lock serializes the capacity decision + write so concurrent signups
+-- can't over-enroll and concurrent cancels can't double-promote. Mirrors the
+-- pure lib/classes-logic rule; this SQL is the runtime authority.
+CREATE OR REPLACE FUNCTION public.class_signup_tx(
+  p_session_id uuid, p_space_id uuid, p_member_id uuid
+) RETURNS TABLE(signup_id uuid, signup_status text, err text)
+  LANGUAGE plpgsql SECURITY DEFINER SET search_path = public AS $$
+DECLARE
+  v_cap int; v_reg int; v_status text; v_id uuid;
+BEGIN
+  PERFORM pg_advisory_xact_lock(hashtext(p_session_id::text));
+  IF EXISTS (
+    SELECT 1 FROM class_signups
+    WHERE session_id = p_session_id AND member_id = p_member_id
+      AND space_id = p_space_id AND status <> 'cancelled'
+  ) THEN
+    RETURN QUERY SELECT NULL::uuid, NULL::text, 'already'::text; RETURN;
+  END IF;
+  SELECT COALESCE(s.capacity, c.capacity) INTO v_cap
+  FROM class_sessions s JOIN classes c ON c.id = s.class_id
+  WHERE s.id = p_session_id AND s.space_id = p_space_id;
+  IF NOT FOUND THEN
+    RETURN QUERY SELECT NULL::uuid, NULL::text, 'no_session'::text; RETURN;
+  END IF;
+  SELECT count(*) INTO v_reg FROM class_signups
+  WHERE session_id = p_session_id AND status = 'registered';
+  v_status := CASE WHEN v_cap IS NULL OR v_reg < v_cap
+                   THEN 'registered' ELSE 'waitlisted' END;
+  INSERT INTO class_signups (session_id, space_id, member_id, status)
+  VALUES (p_session_id, p_space_id, p_member_id, v_status)
+  RETURNING id INTO v_id;
+  RETURN QUERY SELECT v_id, v_status, NULL::text;
+END;
+$$;
+
+CREATE OR REPLACE FUNCTION public.class_cancel_tx(
+  p_session_id uuid, p_space_id uuid, p_member_id uuid
+) RETURNS TABLE(cancelled_id uuid, promoted_id uuid, err text)
+  LANGUAGE plpgsql SECURITY DEFINER SET search_path = public AS $$
+DECLARE
+  v_sid uuid; v_status text; v_cap int; v_reg int; v_prom uuid;
+BEGIN
+  PERFORM pg_advisory_xact_lock(hashtext(p_session_id::text));
+  SELECT id, status INTO v_sid, v_status FROM class_signups
+  WHERE session_id = p_session_id AND member_id = p_member_id
+    AND space_id = p_space_id AND status <> 'cancelled'
+  LIMIT 1;
+  IF NOT FOUND THEN
+    RETURN QUERY SELECT NULL::uuid, NULL::uuid, 'not_signed_up'::text; RETURN;
+  END IF;
+  UPDATE class_signups SET status = 'cancelled' WHERE id = v_sid;
+  v_prom := NULL;
+  IF v_status = 'registered' THEN
+    SELECT COALESCE(s.capacity, c.capacity) INTO v_cap
+    FROM class_sessions s JOIN classes c ON c.id = s.class_id
+    WHERE s.id = p_session_id AND s.space_id = p_space_id;
+    SELECT count(*) INTO v_reg FROM class_signups
+    WHERE session_id = p_session_id AND status = 'registered';
+    IF v_cap IS NULL OR v_reg < v_cap THEN
+      SELECT id INTO v_prom FROM class_signups
+      WHERE session_id = p_session_id AND space_id = p_space_id
+        AND status = 'waitlisted'
+      ORDER BY signed_up_at ASC LIMIT 1;
+      IF v_prom IS NOT NULL THEN
+        UPDATE class_signups SET status = 'registered' WHERE id = v_prom;
+      END IF;
+    END IF;
+  END IF;
+  RETURN QUERY SELECT v_sid, v_prom, NULL::text;
+END;
+$$;
+
 
 -- =============================================================================
 -- 20. Equipment registry + reservations