Restore apps catalog reindexing

[akirk]

Summary

• Restores apps.json after it was regenerated as {}.
• Updates reindex_postprocess.py so the app catalog is built from blueprints/**/blueprint.json entries whose meta.categories include Apps.
• Keeps the catalog generation aligned with Move app blueprints #203, which moved app blueprints out of apps/ and into blueprints/.

Cause

build_apps_index() still walked the removed apps/ directory. With no files there, the deployment bot generated an empty catalog.

Validation

• python3 reindex_postprocess.py --test
• npm run validate:my-wordpress-json