Migrate CI/CD from GCP self-hosted runners to Modal

[baogorek]

Summary

Replace the self-hosted GCP VM runners with Modal serverless compute for CI/CD workflows.

Motivation

• Cost reduction: Pay-per-use vs always-on VM (~$180/month → ~$60-80/month)
• No infrastructure management: No VM maintenance, spot instance handling
• Parallelization potential: Local area builds can run 435 CDs concurrently

Changes Required

• Create Modal app with data_build.py for dataset builds
• Create Modal app with local_area.py for local area publishing
• Update reusable_test.yaml to trigger Modal instead of self-hosted
• Update local_area_publish.yaml to trigger Modal
• Add Modal secrets to GitHub (MODAL_TOKEN_ID, MODAL_TOKEN_SECRET)
• Create Modal secrets (huggingface-token, gcp-credentials)
• Validate in production
• Decommission GCP self-hosted runner VM

Testing

Successfully tested modal run modal_app/data_build.py --test-lite --no-upload locally.