[Request] Generate PMTiles mosaics for LayerSets

[jacobwhall]

Describe the feature or enhancement you're looking for
I would like OIM to additionally provide PMTiles mosaics for LayerSets. This would be amazing for Yesterdays, where we could directly load mosaics from OIM, and edits to OIM would propagate naturally to our platform.

Describe alternatives you've considered
Downstream consumers such as Yesterdays could generate PMTiles ourselves if this is out of scope for OIM.

Additional context
PMTiles are a wonderful target for OIM mosaics: they are capable of storing a high-resolution view of a completed LayerSet while remaining lightweight for users to stream.

[jacobwhall]

It might be possible to generate the PMTiles from the XYZ tiles OIM already generates

[mradamcox]

This would be a great addition to the platform. Here's a quick summary of how it could be implemented, based on existing patterns.

• First, a new method on the mosaicker.Mosaicker class like generate_pmtiles would hold the actual logic to create the output file.
  • These lines in XYZ tileset creation show how an existing COG can be used if present, or how a mosaic VRT is created on demand if not.
• Wire that new method into this management command for testing/development

To hook the working generation method into the new Job management system:

• Wrap it in a celery task like this, register that task in settings.py using the correct exchange
• Update georeference.models.Job's operation field to add a new layerset_to_pmtiles option
• Update Job.run() where a basic if/else checks the operation type and calls the appropriate celery task, created above.

Finally, there would be a bit of frontend stuff to do in the Jobs Svelte component, but the logic I currently have around testing if a COG or tileset is "stale" and displaying its date in red is pretty messy, so this would be a good opportunity to clean that up at the same time.