Separate deleted videos from missing ones #670

Right now every video that's no longer on disk looks the same. A video deleted in Youtarr, one removed by automatic cleanup, and one whose file just disappeared all get the same removed flag and the same red icon on the Videos page. They also stay there forever. The Missing filter can exclude them, but it resets every time the page is opened, and anyone with automatic cleanup turned on builds up a long list of these over time. I've had multiple reports from users who want a way to clear them out, similar to Plex's "Empty trash".

I looked at how a few other apps handle this:

• Plex doesn't track why a file is gone. Anything unavailable gets a trash icon until the trash is emptied, by hand or automatically after each scan.
• Sonarr and Radarr show the episode or movie as "Missing" either way, but record why the file was deleted (Manual, MissingFromDisk, Upgrade) in History.
• Pinchflat sets a culled_at timestamp when its retention cleanup deletes a file.
• Sonarr and Jellyfin both skip their disk scan when the library folder is missing or empty, so an unmounted drive doesn't mark everything as gone.

None of them try to work out why a file went missing. Youtarr can't tell "deleted outside the app" from "the drive died" either, but it does know when it deleted something itself.

Proposed behavior:

• Use two states instead of one:
  • Deleted: Youtarr deleted the file, through a delete in the app or through automatic cleanup. Shown in a neutral style, with the details in the tooltip and video modal (e.g. "Deleted by automatic cleanup on Oct 3 (older than 30 days)").
  • Missing: Youtarr expected a file and couldn't find it. Keeps the red warning style.
• Record a reason and a timestamp on the Videos row when a video is removed. The reasons are deleted in Youtarr, automatic cleanup (with the rule that removed it), not found on disk, and marked as deleted.
• On the Videos page, deleted videos are hidden by default, with a "Show deleted" filter that's remembered in the browser. Missing videos still show.
• Add a "Mark as deleted" action for missing videos, for selected ones or all of them. They move to Deleted and drop out of the default view.
• Channel and playlist pages show "Deleted" or "Missing" on previously downloaded videos and keep listing both, since that's where someone would go to re-download one.
• Re-downloading works the same as today. Deleted and missing videos stay in complete.list, so they don't download again automatically.
• If a deleted or missing video's file shows up again, the file check and rescan mark it as downloaded again, like they do now.
• Videos removed before this change have no recorded reason. Show them as Deleted ("Removed before Youtarr recorded why") rather than filling the page with Missing.

Additional related enhancement that will be addressed for this issue:

• The playlist pages that show lists of playlist videos do not currently support showing missing videos like the channel video and video library listings. I will also add proper support on the playlist page for showing the new Deleted/Missing status for videos there.

Protecting against an unmounted share:
This is already a problem today. The Videos page checks each listed file when it loads and saves the result. If a NAS share is unmounted and its mount point is empty, just opening the page marks those videos missing, and the next rescan marks the rest. They come back once the share is mounted again, but "Mark as deleted" on all missing videos would make it easy to bury the whole library during an outage. Before marking anything missing, Youtarr should check that the downloads folder (and each __subfolder) exists and isn't empty. If it doesn't, Youtarr should skip the check and show a warning.