Add `preview:queue-all` command to queue all media files for the background job

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Feature request

Which Nextcloud Version are you currently using: not relevant

Is your feature request related to a problem? Please describe.
Now that we have a background job to handle the pre-generation, it would be nice to be able to queue all media files for the job to handle them.

Describe the solution you'd like
Add a preview:queue-all command that queues all media files.

Describe alternatives you've considered
Do not implement the command. An admin may very well run the preview:generate-all initially after installing the app or at any later point. Queuing all images will cause congestion and new previews will likely not be generated for a long time.

Additional context
none

Start by locating the existing preview:generate-all command and the background job it uses. Trace how media files are discovered and queued, then verify that preview:queue-all queues every media file without generating previews immediately.