feat(devops): introduce terraform infrastructure as code

[Code-Paragon]

Summary

Transitioned AgenticPay from manual/untracked infrastructure to a robust Infrastructure as Code (IaC) setup using Terraform. This standardizes our deployment pipeline across AWS.

Closes #68

Changes Made

• Defined All Resources: Created modular infra/main.tf establishing a secure VPC, an ECR + AWS App Runner configuration for the Express.js backend, and AWS Amplify definitions for the Next.js frontend.
• Multi-Environment Support: Created explicit .tfvars configurations for dev, staging, and prod networks, dynamically mapping to the appropriate Stellar networks (Testnet vs. Public).
• State Management: Implemented an S3 backend with DynamoDB locking to ensure team collaboration is safe and state is securely encrypted.
• Documentation: Added comprehensive infra/README.md detailing usage, prerequisites, and deployment commands.

Validation

• Ran terraform fmt and terraform validate to ensure HCL correctness.
• Infrastructure is completely isolated via workspace variables.

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