Scheduled registry maintenance job

[DropSnorz]

Create a monthly maintenance job using GitHub Action to check metadata and URL validity for the latest version of each package. The idea is to catch potential invalid digests or broken urls. Focusing on the latest version avoids revalidating the entire registry.

It's a feature I originally wanted to develop in owlplug-registry: OwlPlug/owlplug-registry#49

The maintenance job will check plugins, presets and projects fields:

• audio - Validate url is not dead
• files[*].url - Validate url is not dead
• files[*].sha256 - Validate that the digest still match the file hash (relevant ? requires to download the full file)
• image - Validate that url is not dead (and image meet some size criterias like max size ?)
• url - Validate that url is not dead

The report can be published as a Github issue.

	Message	Value	Manifest
🚫	Invalid return code 500 for url	https://github.com/airwindows/airwindows/archive/refs/heads/master.zip	plugins/airwindows/airwindows/1.0.0/index.yaml
⚠	Invalid return code 404 for audio	https://open-audio-stack.github.io/open-audio-stack-registry/plugins/airwindows/airwindows/airwindows.flac	plugins/airwindows/airwindows/1.0.0/index.yaml

[kmturley]

Bonne idee!

The GitHub Action in validate.yml already does some of this work:
https://github.com/open-audio-stack/open-audio-stack-registry/blob/main/.github/workflows/validate.yml

Likely whatever monthly validation should also be done on PRs. So we can share the same code. Just run at different times.

The current Action only scans files that have changed. Could create a duplicate Action without the changed files step.
Adding a schedule cron timing: https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#schedule