Request: Allow Downloaded Episodes to Be Kept Even When No Longer Officially Available

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Is your feature request related to a problem? Please describe.
Some podcasts I follow only have the most recent few months of episodes in their feed, e.g. this one. I had downloaded episodes from early March (when I started using Podverse), but those downloads have disappeared and the remaining downloads are only of episodes that are still in the official feed, which are from late March to present.

Describe the solution you'd like
I'd like for downloaded episodes to be kept regardless of whether they're still available in the official podcast feed.