fix: improve period unit ts type

[itsramiel]

Thank you for contributing to purchases-hybrid-common. Before pressing the "Create Pull Request" button, please provide the following:

• A description about what and why you are contributing, even if it's trivial:
  If periodUnit can be only DAY, WEEK, MONTH or YEAR as mentioned in the comment, then it is better to type it that way. Makes it easier to switch on the predefined union or perform if checks with intellisense

• The issue number(s) or PR number(s) in the description if you are contributing in response to those:
  There is no issue number

• If applicable, unit tests.
  Not applicable