T4: Add Postman API testing + CI workflow

[jag2430]

Add Automated API Testing and CI Pipeline Enhancements

Overview

This PR introduces a complete API testing workflow and upgrades our CI pipeline to meet the project requirements for automated testing, style checking, static analysis, and coverage reporting.

Changes Made

• Postman API test suite added under postman/

  • postman_collection.json
  • postman_environment.json

• Newman-based API testing job added to GitHub Actions

  • Spins up Postgres
  • Builds and runs the Spring Boot service
  • Executes the full Postman collection
  • Generates HTML + JUnit reports

• Updated CI pipeline (.github/workflows/ci.yml) now runs:

  • Unit tests
  • Integration tests
  • API tests (Newman)
  • JaCoCo coverage reporting
  • Checkstyle and PMD static analysis
  • Build summary aggregation

Summary

This PR completes the requirements for automated API testing and continuous integration:

• Validates all endpoints
• Enforces style and static analysis tools
• Confirms data flow across controllers, services, and the database
• Provides fully automated CI feedback on every PR