Enhancement: Add filter criterion for subscription state

I'd like to show, for example, any Downloaded, In Progress episodes irrespective of subscription state.

Proposal: Create a new filter that allows for podcast subscription state to be defined ( _All_ | _Subscribed_ | _Not Subscribed_)

Since current behavior is that a filter automatically includes only episodes of subscribed podcasts, perhaps the above criterion can default to _Subscribed_ when presented in the UI so that existing filters remain backwards compatible, and are updated only if the user changes the value to include _All_ or _Not Subscribed_.

Start by tracing the existing episode filter flow and the UI that selects filter criteria. Check how subscription state is currently applied, then define how All, Subscribed, and Not Subscribed interact with Downloaded and In Progress filters. Done means existing filters retain Subscribed behavior by default while each new option produces the requested episode set.

Written by the indexing model from the issue text.