Ensure MustGetSortOption returns a default bson.D{{"_id", -1}} on parse errors and
GetPaginationPipeline uses {_id: -1} when sort is nil to provide consistent default ordering.
- Deleted the db module, consolidating database-related functionality into the core/mongo package for better organization and maintainability.
- Updated all import paths across the codebase to replace references to the removed db module with core/mongo.
- Cleaned up unused code and dependencies, enhancing overall project clarity and reducing complexity.
- This refactor improves the structure of the codebase by centralizing database operations and simplifying module management.
