Adding backend tests

[Polliog]

This pull request introduces a comprehensive continuous integration (CI) workflow, testing infrastructure, and performance/load testing setup for the backend. It adds GitHub Actions CI, Docker Compose configuration for test environments, backend test environment variables, and a full load testing suite using k6. Documentation and status badges are also updated to reflect these improvements.

CI & Testing Infrastructure:

• Added a new GitHub Actions workflow (.github/workflows/ci.yml) that runs backend tests with coverage, performs TypeScript checks, builds Docker images, and includes a deploy-to-staging step for the main branch. The workflow also enforces a minimum 70% code coverage threshold and uploads results to Codecov.
• Introduced a dedicated Docker Compose file (docker-compose.test.yml) to spin up isolated test services for Postgres, Redis, MailHog, and the backend, with appropriate health checks and optimized settings for testing and load scenarios.
• Added a .env.test file for the backend, specifying environment variables tailored for test runs, including database, Redis, API key, SMTP, and rate limiting configuration.

Performance & Load Testing:

• Added a load-tests directory to the backend with a detailed README.md explaining how to run performance and load tests using k6, including scenarios, targets, and troubleshooting tips.
• Implemented a k6 load test script (ingestion.js) that simulates various log ingestion scenarios (sustained, burst, ramp-up), collects latency and error metrics, and outputs a summary report.

Documentation & Status Badges:

• Updated the main README.md to include CI and code coverage badges, providing visibility into build and test status.