Introduce a template-based system for shipment checklists to automate
the creation of required inspection items upon shipment approval.
- Add `ShipmentChecklistTemplate` model and migration to define reusable
checklist structures.
- Extend `ShipmentChecklist` schema to support item titles, requirement
flags, sorting, and attachment paths.
- Implement `instantiateChecklist` in `ShipmentReviewService` to
automatically generate checklist items from active templates based on
shipment direction (import/export).
- Refactor `ShipmentChecklistResource` and `ChecklistsRelationManager`
in Filament to support the new data structure and improved UI.
- Update `ShipmentChecklist` model with helper methods for completion
tracking and improved relationship handling.
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