Add Terraform setup for ECS Express Mode

[Subhransu-De]

What to build

Add a Terraform infrastructure setup under /infra that deploys this FastAPI template using Amazon ECS Express Mode. This should be the fastest AWS-native container deployment path for a one-shot FastAPI app, using an existing container image and letting ECS provision the Fargate service, HTTPS endpoint, load balancing, autoscaling, networking, logs, and supporting resources.

Use the AWS ECS Express Mode guidance as the architecture reference.

Acceptance criteria

• Terraform can provision an ECS Express Mode service for the FastAPI app.
• The setup supports configuring the container image, service name, CPU, memory, port, and health check path.
• The app receives non-secret environment variables through Terraform variables.
• Secrets can be sourced from AWS Secrets Manager or SSM Parameter Store.
• CloudWatch logging is enabled or uses the ECS Express Mode default logging behavior.
• Outputs include the deployed service URL and service identifier.
• Documentation explains required AWS permissions, variables, deploy command, destroy command, and expected costs/resources.

Blocked by

None - can start immediately.