Add gharts RDS instance and IRSA role to 01_infra

[afrittoli]

Provisions the AWS infrastructure required by the gharts service on the arc dev cluster.

This PR

• RDS db.t3.micro PostgreSQL instance with IAM authentication enabled
• Security group allowing PostgreSQL access from within the VPC
• DB subnet group covering private subnets
• IAM role and rds-db:connect policy for IRSA
• Outputs: gharts_rds_host, gharts_irsa_role_arn (consumed by 02_helm)

Stack Overview

1. PR #??? (this PR): 01_infra — RDS instance and IRSA role ← MERGE FIRST
2. PR Add gharts to the arc dev cluster #404: 02_helm — gharts Helm release (depends on this PR)

Review Guidance

Changes are isolated to 01_infra/gharts-rds.tf and 01_infra/outputs.tf.

[linux-foundation-easycla]

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