Playground App Stack

Terraform stack for the Playground API/Blazor app. Uses shared modules from ../../modules.

• Env/region stacks live under envs/<env>/<region>/ (backend.tf + *.tfvars + main.tf).
• App composition lives under app_stack/ (wiring ECS services, ALB, RDS, Redis, S3).
• Images are built from GitHub Actions, pushed to ECR, and referenced in tfvars.