- Consolidated item creation and update validation into a common function to reduce code duplication.
- Updated the `validate_create` and `validate_update` functions to utilize the new common validation logic.
- Introduced a new `common_validators.py` file for shared validation functions.
- Added a new `fractions.py` file to handle fraction-related logic and searches.
- Enhanced the `LineCustom` model to use an enumeration for `fraction_type`.
- Improved the `ItemService` class with methods for locking invoices and renumbering line items.
- Updated the `Sector` model to use a boolean type for the `authorized` field.
- Fixed import issues in the router by replacing the old `a24_router` with `sitar_router`.
