Implements SaveCategory, SelectCategory, SelectCategoriesOfSite,
SelectCategoryBySlugAndSite, DeleteCategory, SelectCategoriesOfPost,
SelectPostsOfCategory, CountPostsOfCategory, and SetPostCategories on
the DB Provider, along with BeginTx/QueriesWithTx for transaction
support. Also fixes pre-existing compilation errors in provider_test.go
(missing PagingParams args) so new tests can compile and run.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Leon Mika