Implement a new pricing architecture based on a 4-quadrant model (Export/Import
x Document/Non-Document) to replace the previous 2-zone model. This includes
the ability to handle different zones for parcels and documents per country.
- Add `RateEntry` and `CountryZone` models for managing complex rate matrices.
- Implement `RateImportService` to parse Excel templates using PhpSpreadsheet.
- Add `RateImportResource` in Filament for administrative rate uploads.
- Refactor `PriceCalculatorService` to utilize the new 4-quadrant lookup logic.
- Add database migrations to extend `rate_entries` and create `country_zones`.
- Update `DEPLOYMENT.md` with new architectural details and deployment steps.
