Increase GCP App Engine resources

[anth-volk]

Summary

• increase App Engine flex CPU from 2 to 4 for staging and production
• increase App Engine flex memory from 8GB to 16GB for staging and production
• increase Gunicorn workers from 2 to 4 to match the CPU allocation

Notes

• leaves max_num_instances at 1
• targets the GCP maintenance branch, not main/Modal

Validation

• git diff --check