@spec invite(Plausible.Site.t(), Plausible.Auth.User.t(), String.t(), atom()) ::
  {:ok, Plausible.Auth.Invitation.t()}
  | {:error, Ecto.Changeset.t()}
  | {:error, :already_a_member}
  | {:error, {:over_limit, non_neg_integer()}}
  | {:error, :forbidden}

Invites a new team member to the given site. Returns a %Plausible.Auth.Invitation{} struct and sends the invitee an email to accept this invitation.

The inviter must have enough permissions to invite the new team member, otherwise this function returns {:error, :forbidden}.

If the new team member role is :owner, this function handles the invitation as an ownership transfer and requires the inviter to be the owner of the site.

@spec stats_start_date(Plausible.Site.t()) :: Date.t() | nil

Returns the date of the first event of the given site, or nil if the site does not have stats yet.

If this is the first time the function is called for the site, it queries Clickhouse and saves the date in the sites table.