v1.0.2: fix: Align queue callback parameters with PHP 8 named arguments

On PHP 8, `adt/background-queue` calls job callbacks using named parameters. If the callback's parameter names do not precisely match the keys under which the job was published, PHP throws an "Unknown named parameter" error. This caused background export jobs to get stuck in an infinite retry loop.

This change updates the `processExport` callback to accept `exportId` directly as a named parameter, and introduces a constant for the publishing key to ensure consistency. A new test verifies this critical naming alignment and type.