infra: bootstrap arco-testing-dev and make background automation configurable

[ethan-tyler]

Summary

• update ownership metadata and bootstrap the arco-testing-dev Terraform environment
• align IAM and Cloud Run Terraform wiring for the test environment
• include the initial testing-infra baseline used by later cloud worker work

Test Plan

• terraform -chdir=infra/terraform init -backend=false -input=false
• terraform -chdir=infra/terraform validate
• cargo run -p arco-api --bin gen_openapi > crates/arco-api/openapi.json
• cargo test -p arco-api --test openapi_contract -- --nocapture