feat(infra): Add complete main.tf, variables.tf, outputs.tf for Terraform module

[Moonwalker-rgb]

Summary

Resolves #164 – Refactor infrastructure/terraform/ from flat .tf files into a composable module structure.

Changes

• Create main.tf: Root module entry point orchestrating network, compute, and data submodules plus IAM, CloudFront, WAF, Route53, ACM, CloudWatch, SNS, and Secrets Manager resources
• Create outputs.tf: Surfaces ARNs for every created resource (60+ outputs)
• Consolidate variables.tf: Single-source-of-truth, removing 12 duplicate variable declarations
• Create modules/network: VPC, subnets, route tables, NAT gateway, NACLs, security groups
• Create modules/compute: EKS cluster + node groups, ALB + target groups + listeners, ASG + launch templates
• Create modules/data: RDS PostgreSQL, ElastiCache Redis, S3 buckets (uploads/backups/logs/frontend), backup vaults + KMS keys + plans
• Archive old flat files: 43 previous .tf files moved to archive/ for migration reference
• Update env tfvars: All 3 environments updated with new variables

Acceptance Criteria

• Root module (main.tf, variables.tf, outputs.tf) created
• Submodules split into network, compute, data
• Outputs surface ARNs of every created resource
• Fixes duplicate resource definitions (e.g., aws_security_group.alb defined in both alb.tf and security-groups.tf)
• Fixes duplicate variable declarations across 12 files
• Properly passes aws.dr provider alias to data module for cross-region backup