Reimplementing / reactivating file hash checker from old JMM server

[t0815]

Suggestion

Im wondering, if it would be possible, we could get back the file-verification feature back. It was actually quite handy to find
damaged/corrupted files.

[da3dsoul]

The code actually still exists iirc. What it needs is UI, api, and cleanup, mostly.

[ElementalCrisis]

Linking #555 to this for the eventual code cleanup.

This is the only screenshot I apparently have of it. But I remember how it works.

Some notes on the utility.

• Can be expanded so it's similar to File Search, user can select specific series, last 100 files or their entire collection if they so choose.
• As this is a background task, user will be notified when done, for now maybe a persistent toast notification that has to be manually closed.
• Output is a filterable list, passed, failed, unknown.
• Output can be exported.

I'll add the UI work to my list.