@spec find(String.t() | Plausible.Billing.Subscription.t(), [
  Plausible.Billing.Plan.t()
]) ::
  Plausible.Billing.Plan.t() | nil

@spec find(nil, any()) :: nil

Finds a plan by product ID.

Returns nil when plan can't be found.

@spec for_user(Plausible.Auth.User.t()) :: [Plausible.Billing.Plan.t()]

Returns a list of plans available for the user to choose.

As new versions of plans are introduced, users who were on old plans can still choose from old plans.

@spec site_limit(Plausible.Auth.User.t()) :: non_neg_integer() | :unlimited

Returns the limit of sites a user can have.

For enterprise customers, returns :unlimited. The site limit is checked in a background job so as to avoid service disruption.

@spec suggest(Plausible.Auth.User.t(), non_neg_integer()) ::
  Plausible.Billing.Plan.t()

Returns the most appropriate plan for a user based on their usage during a given cycle.

If the usage during the cycle exceeds the enterprise-level threshold, or if the user already belongs to an enterprise plan, it suggests the :enterprise plan.

Otherwise, it recommends the plan where the cycle usage falls just under the plan's limit from the available options for the user.

@spec yearly_product_ids() :: [String.t()]

List yearly plans product IDs.