This commit introduces a comprehensive suite of unit tests for the application's models, repositories, and services, achieving 100% test coverage for all new and modified files.
Key changes include:
- Added unit tests for all services in `internal/app`.
- Added unit tests for all repositories in `internal/data/sql`.
- Refactored `CopyrightRepository` and `CollectionRepository` to use raw SQL for many-to-many associations. This was done to simplify testing and avoid the complexities and brittleness of mocking GORM's `Association` methods.
- Removed a redundant and low-value test file for domain entities.
- Fixed various build and test issues.
- Addressed all feedback from the previous code review.
