feat(billing): cap active users on an owner subscription

Super admin can set a seat limit of 3, 10, 20, or a negotiated number so an owner cannot add users past that cap.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/prisma/migrations/20261102183000_owner_user_cap/migration.sql

@@ -0,0 +1 @@
+ALTER TABLE "BillingAccount" ADD COLUMN "maxUsers" INTEGER;