Fix Plane S3 upload redirects

[WhiteRow33]

Summary

• add CORS configuration for the Plane doc-store S3 bucket
• default Plane's S3 endpoint to the regional AWS S3 endpoint to avoid global endpoint 307 redirects
• update README storage/runbook notes for the regional endpoint and availability/isolation context

Validation

• terraform fmt -recursive terraform
• terraform -chdir=terraform/environments/prod init -backend=false -input=false
• terraform -chdir=terraform/environments/prod validate
• git diff --check

Note: local backend-backed Terraform init/plan was not run because Terraform could not consume the local AWS CLI login credential chain for the S3 backend; CI should run the normal backend-backed plan/apply.