Refactor out ArchiveExtract Methods into core service methods

#40
- [x] Create 'Core Service' class with an interface to be shared with other Extract Worker services

- [x] Refactor methods into 'Core Service'

- GetJob (string jobname) returns (job, List JobItem)

- GetLastFulfilment() returns (Is Success, Error, Fulfillment)

- CreateFulfillmentSaga(params) returns(Is Success, Error, Fulfillment, List Fulfillment Item)

- UpdateFulfillment(Fulfillment) returns (Fulfillment)

The issue describes moving methods from an ArchiveExtract worker into a shared core service. Look for the existing ArchiveExtract service to understand the current implementation. The new core service needs to expose the listed methods (GetJob, GetLastFulfilment, CreateFulfillmentSaga, UpdateFulfillment). Check how other extract workers would use this interface. Verify the refactor doesn't break existing scraping or data preservation flows.

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