Add Terraform AWS infrastructure and enhanced CI/CD pipeline (FRE-4574)

- Terraform modules: VPC, ECS Fargate, RDS PostgreSQL, ElastiCache Redis, S3, Secrets Manager, CloudWatch
- Multi-environment support: staging and production configs
- ECS auto-scaling: CPU-based scaling with configurable min/max
- CI/CD: pnpm caching, Docker Buildx, Trivy security scanning, Terraform plan on PR
- Deploy: ECS service updates with automatic rollback on health check failure
- Backup: automated RDS snapshots, S3 versioning, ElastiCache snapshots
- Monitoring: CloudWatch dashboards, CPU/memory/5xx alarms
- Rollback script for manual service rollback
- Infrastructure documentation with architecture overview

infra/environments/production/terraform.tfvars.example

@@ -0,0 +1,4 @@
+hibp_api_key     = "YOUR_HIBP_API_KEY"
+resend_api_key   = "YOUR_RESEND_API_KEY"
+sentry_dsn       = "YOUR_SENTRY_DSN"
+datadog_api_key  = "YOUR_DATADOG_API_KEY"