Add multi-region AWS deployment scripts with Terraform

[Wilfred007]

What

Terraform modules exist for single-region AWS deployment. Add multi-region support for high availability and disaster recovery.

Why

Single-region deployment means regional AWS outage takes down the entire payroll system. Multi-region is required for production SLA.

Scope

In scope: Multi-region VPC, cross-region RDS replication, Route53 failover
Out of scope: Multi-cloud

Acceptance Criteria

• Infrastructure deployed in 2+ AWS regions
• Database replicated across regions
• DNS failover configured
• Failover tested and documented

Technical Context

• infrastructure/terraform/modules/ — existing modules
• infrastructure/terraform/environments/ — staging, production