Add seen_by_guid property to feeds for dynamic link feeds

[StormPooper]

I recently moved from Jackett to Prowlarr and noticed that some torrents were being re-added to my download client despite entries being added to my seen_file. After some investigation. I realized it was because Prowlarr "proxies" the download link to go through itself rather than directly to the result and in doing so generates a unique URL. The problem was that this URL is not static and changes for each request, so my seen_file was never matching on the URL and bloating with hashes for the same torrent over and over.

To fix this, I've added an option called seen_by_guid to the feed object. This, as the name implies, will use the guid on the feed item, which should be consistent. This defaults to false to prevent invalidating existing seen files and falls back to the link if there is no guid element in the result.

I've also added tests for the areas I've impacted to try and ensure I didn't break anything. All tests pass and it seems to be behaving correctly in Docker with and without seen_by_guid from my manual testing.

Let me know if you'd like any changes, and thank you for creating transmission-rss.

[nning]

Looks perfect on the first glance! Thanks for your contribution! I will test and probably merge in the next days.

[StormPooper]

No worries, thanks for considering it. Let me know if you want me to change anything 👍

[nning]

Thanks again for your contribution! I had a chance to have a second look now and I will merge it directly.