feat(previews): improved logging for movie previews

[invario]

Improved/additional logging when generating previews for video/movie files at INFO level and always log ffmpeg output to DEBUG (not only when ffmpeg fails/errors).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[github-actions]

Hello there,
Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!

(If you believe you should not receive this message, you can add yourself to the blocklist.)

[invario]

@provokateurin If you have the time, any chance of taking a look at this please?

[provokateurin]

Sorry yes, it somehow slipped past me!

[provokateurin]

LGTM, just some minor changes I would do to avoid spammy logs.

[invario]

LGTM, just some minor changes I would do to avoid spammy logs.

Thanks, pushed changes!

[provokateurin]

/backport to stable32