Introduce a shipment status history system to audit status changes,
including the reason for change and the user responsible. This includes
a new `ShipmentStatusHistory` model and a bulk action in the Filament
ShipmentResource to transition statuses.
Enhance the rate import workflow by adding a downloadable Excel
template and refactoring the `ImportRatesPage` to use the modern
Filament form schema.
- Add `ShipmentStatusHistory` model and migration
- Add bulk `changeStatus` action to `ShipmentResource`
- Add `source` field to tracking events
- Implement `ShippingRatesTemplateExport` for rate template downloads
- Refactor `ImportRatesPage` form implementation and UI
- Add `statusHistories` relationship to `Shipment` model
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