Refactor/restructure to monorepo

[mgrablo]

This pull request primarily improves project structure and developer experience by refining configuration files, updating the test workflow, and adding helpful documentation and ignore rules. The changes ensure consistency across environments, make backend testing more robust, and streamline local development.

Project and build configuration:

• Added rootProject.name = "biblionode-api" in settings.gradle.kts to explicitly set the project name for Gradle builds.
• Updated .gitattributes to enforce LF line endings for .sh scripts and Dockerfile, improving cross-platform consistency.

CI/CD and testing workflow:

• Refactored the GitHub Actions workflow to rename the job to "Test backend", set the working directory to biblionode-api, and updated JaCoCo badge generation to use the correct path. This ensures tests and coverage reports run in the appropriate context. [1] [2]

Developer experience and documentation:

• Added a .gitignore to biblionode-api to exclude build artifacts, IDE files, and environment files, reducing clutter and preventing accidental commits of sensitive or unnecessary files.
• Improved the README.md with instructions to navigate to the API directory before running backend commands, clarifying setup steps for new contributors.