Introduce Cancel Export Endpoint to Core API

🔍 Problem Statement
Currently, once an export process has been initiated, there is no way to cancel it mid-process. This creates operational inefficiencies when users start exports by mistake, change requirements, or realize the data set is too large or unnecessary. Without cancellation, system resources are wasted and users must wait for irrelevant exports to complete.

💡 User Story
As a developer,
I want the ability to cancel an in-progress export by providing its UUID,
so that I can stop unnecessary processing, free up resources, and avoid waiting for irrelevant or mistakenly initiated exports to complete.

🎯 Definition of Done (DoD)

• A new “Cancel Export” Core API endpoint exists, requiring UUID as a parameter.

• Logic ensures:

  • The batch related to the UUID is found and cancelled.

  • The related export is located.

  • All associated export_chunks are marked as finished.

  • The export itself is marked as finished and soft-deleted.

• Endpoint is covered with unit and integration tests.

• Proper error handling and response messages are implemented (e.g., invalid UUID, already finished export).

• API documentation is updated to include usage details and examples.