#15 implement holidays overview endpoint

[mohsenk]

#15 implement holidays overview endpoint

Description

Implement an endpoint for the holiday overview. We need a list of imported holidays grouped by country and year so the front end can filter the data.

Changes

• Implement an endpoint to retrieve an overview of imported holidays.
• Add countryCode to the fetch endpoint.

Checklist

• Code adheres to the project’s back-end guidelines.
• Unit tests cover new functionality.
• Integration tests are added or updated.
• Exception handling is implemented consistently.
• Logging is added for critical operations.
• Database migrations (if applicable) are included and tested.
• API documentation is updated if necessary.
• Changes are reviewed for potential performance impacts.
• Security vulnerabilities are assessed and mitigated.
• Dependencies are updated or verified as compatible.
• Code is reviewed for readability and maintainability.
• Environment configurations (if any) are documented.
• Endpoints are tested for expected behavior.

Related Issues

Link to any related issues: #15