Get current user subscription for an installed app

Returns paid entitlement for the authenticated user for the app identified by appUuid. The OAuth access token must belong to that app.

When the authenticated user is an agency team member, the response also includes a `managedCreators` array containing the subscription state for every creator the team member is assigned to. The agency user's own top-level subscription will be `status: "none"` (agency team members do not install apps themselves). For non-agency users `managedCreators` is an empty array.

A `status: "pending"` entry with `currentPeriodEnd: null` is a checkout awaiting payment: the user is not entitled to the plan yet, so `planUuid` and `planName` are `null` until the payment settles. A `pending` entry with a non-null `currentPeriodEnd` is a paid subscription with an in-flight cancellation and keeps its plan fields.