feat(model_manager): Add scan and delete of orphaned models

[lstein]

Summary

This PR addresses the situation in which there are "orphaned" models in the INVOKEAI_ROOT/models directory that are not registered in the database. The PR offers two ways of dealing with them:

1. A command line tool named scripts/remove_orphaned_models.py that lists the orphans and prompts the user to delete the orphan model directories.
2. A new "Sync Models" button in the frontend's Model Manager tab that does the same thing interactively.

This functionality complements the recently-added PR #8801 , which implemented a filter to show models in the database that are missing or incomplete, and offers to remove them from the database.

Related Issues / Discussions

None

QA Instructions

Because this functionality has the potential to delete models on disk, you might want to set up a testing invoke root.

Testing the command-line script:

Note that this is intended to be a developer-facing utility and is not particularly polished.

1. Run python3 scripts/remove_orphaned_models.py --root /path/to/INVOKEAI_ROOT
2. Verify that it shows zero orphaned models (assuming you have a clean root)
3. Add some dummy models to INVOKEAI_ROOT:
   cd INVOKEAI_ROOT/models
   mkdir -p orphan1 orphan2/encoder orphan3
   touch orphan1/test.safetensors orphan2/encoder/test.bin orphan3/pytorch.safetensors
4. Run the script again. Now it should report the 3 orphaned models and ask if you wish to remove them.
5. Type 'Y' to delete and verify that the directories are gone.

Testing the UI:

1. Add some dummy models to INVOKEAI_ROOT as described in the previous section.
2. Navigate to the Model Manager; note a new button at the top labeled "Sync Models"
3. Verify that the tooltip is sufficiently informative for a typical user.
4. Press the button and verify that it open a dialogue that correctly lists the orphan models.
5. Toggle open the model names and verify that each shows at least one model file
6. Press the "Delete" button and verify that the model directories are gone.
7. Run "Sync Models" again and verify that it reports no orphans found.

Merge Plan

Simple merge.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• X] Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• [] Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)