Batch generation

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Feature request

Which Nextcloud Version are you currently using: (see administration page)
Nextcloud Hub 10 (31.0.6)

Is your feature request related to a problem? Please describe.
I am using Nextcloud for a private cloud with a low resource server; the preview generation for the whole filesystem would actually saturate the machine resources

Describe the solution you'd like
I would propose to add a parameter to limit the amount of previews generated for each iteration, like
preview:generate-all --num=5000 to elaborate at most 5000 files

Describe alternatives you've considered
Halting the generation manually after some minutes

Additional context
none

Start by locating the preview:generate-all command and reading how it iterates over files and generates previews. Add support for limiting each invocation to the requested number of files, then verify that --num=5000 stops after at most 5000 files while preserving existing behavior without the option.