Add Control Plane deployment configuration

[justin808]

Context

The core goal of this repo is a side-by-side comparison of Kamal + Terraform vs Control Plane + Terraform for deploying Rails 8 to GCP. The Kamal path is complete — now we need the Control Plane path.

Tasks

• Create terraform-cpln/ directory alongside terraform-gcloud/
• Write Terraform config for Control Plane resources (GVC, workloads, etc.)
• Configure Control Plane deployment using cpln CLI or cpflow gem
• Set up the same Rails app to run on Control Plane (same DB, same secrets pattern)
• Add a stand-up/tear-down script parallel to the Kamal one
• Document the setup process in docs/
• Update README with Control Plane deployment instructions

Architecture Decisions

• Same Rails app, same PostgreSQL (Cloud SQL or Control Plane managed)
• Same secret management pattern (adapted for Control Plane)
• Goal: demonstrate the difference is purely in infrastructure/deployment, not app code

Notes

This is Phase 2 of the project. Depends on Phase 1 being verified (issue: Verify end-to-end Kamal deployment cycle).