If shell_exec()/exec() is not available allow using API methods to manage archives

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On some systems it's not possible to use shell_exec()/exec() to start the import. We can provide an alternative by providing a way to initialize imports via API methods.

This would require:
- an API method to import entities
- an API method to start an import for a day and when finished invoke the next day if the daily API request limit hasn't been reached
* we only do one day at a time to avoid any server side request timeouts
- if a setting is set or we can't run shell_exec/exec, use the API method in Tasks.php & Importer.php

Start by reading Tasks.php and Importer.php, focusing on how imports are currently started through shell_exec()/exec(). Trace the import flow and identify where API methods would need to initialize entity imports and process one day at a time. Done means API-based importing works when configured or when shell execution is unavailable, while respecting the daily request limit.

Written by the indexing model from the issue text.