Add "set this image as the cover thumbnail" option to the reader

[Difegue]

This option would allow users to fix the few edge cases where the server doesn't detect the proper cover image in the archive.

This doesn't necessarily need an API endpoint and could just be glued to the Reader Controller class, but being a good host to third-party clients is important!

[Difegue]

Basic API draft:

• GET /api/archives/:id/thumbnail gets two new optional query parameters: no_fallback and path
  • path, like its /api/archives/:id/page equivalent, allows you to get the thumbnail for a specific page of the archive.
    • (This solves Poor performance/usability for editing archives #520 and enables Add an option to show the tags/thumbnail overlay first when opening the Reader #514 )
    • I'm likely going to start running Minion jobs on archive discovery+Reader open to preemptively create thumbnails for every page, so this endpoint should be fast enough.
  • no_fallback allows you to disable the current behavior where a placeholder "No thumbnail" image is returned if your thumb isn't ready, giving you instead a proper error or minion job status.
    • { "success": "0/1", "job_id": "2344", "error": "blerb" }
    • Should this still return an image if the thumb is ready? There are no other endpoints that serve thumbs so you'd have to re-call the URL without no_fallback which feels a bit dumb, but having an endpoint that can give JSON or image data feels a bit schizophrenic
• PUT /api/archives/:id/thumbnail?path=xxx allows you to specify a given thumbnail to be the registered cover.
  • This solves, well, this issue. 😎

[Difegue]

The new/updated APIs are detailed here:
https://sugoi.gitbook.io/lanraragi/v/dev/api-documentation/archive-api#get-archive-thumbnail
https://sugoi.gitbook.io/lanraragi/v/dev/api-documentation/archive-api#update-thumbnail