Create a brand kit

Create one mutable account-owned brand kit. Logos and fonts are uploaded through the kit's asset route after creation. The account plan sets how many kits the account can keep: 1 on Free, a pack-only account, and Starter, 5 on Growth, and 20 on Scale. Every kit of the account counts, archived kits too. A kit over that limit answers 409 brand_kit_limit with error.plan_limit. An account above its limit after a downgrade keeps every kit and can still change and archive them.