[Enhancement] Redownload new media after a delay (#173)

* Added redownload-related columns

* Added methods for fetching re-downloadable media items

* Filled out redownload worker + tests

* Added redownload worker to config.exs cron

* Added to UI and README

config/config.exs

@@ -52,7 +52,8 @@ config :pinchflat, Oban,
     {Oban.Plugins.Pruner, max_age: 30 * 24 * 60 * 60},
     {Oban.Plugins.Cron,
      crontab: [
-       {"@daily", Pinchflat.Downloading.MediaRetentionWorker}
+       {"0 1 * * *", Pinchflat.Downloading.MediaRetentionWorker},
+       {"0 2 * * *", Pinchflat.Downloading.MediaRedownloadWorker}
      ]}
   ],
   # TODO: consider making this an env var or something?