Changing add-ons on a USER subscription applies end to end - PATCH /stripe/subscriptions/addons
reaches Stripe AND the result is written back to UserPremium.addons and pushed to the assigned
server.
Without this string, do not enable Apply on a user subscription: the charge goes through and nothing is delivered.
Capabilities this version of the package knows the names of.
Deliberately widened with
string & {}where it is used, so a NEWER API announcing a capability this package has never heard of still type-checks. A closed union would make an older client reject a newer server, which is the opposite of what a capability list is for.