fix(importer): NZB cleanup behavior and persistence improvements

[drondeseries]

This PR introduces configurable NZB cleanup behavior and improves NZB persistence reliability.

Changes

• Configurable Cleanup: Added nzb_cleanup_behavior setting to choose between delete (default) or keep for both success and failure scenarios.
• Persistence Fix: Ensures NZB files are moved to persistent storage (.nzbs/) immediately upon queue addition to prevent data loss on container restarts.
• Failure Handling: Moves failed NZB files to .nzbs/failed instead of deleting them (unless configured to delete), aiding in debugging.
• Refactor: Aligned persistence logic in service.go with upstream patterns.

Configuration

New config.yaml options:

import:
  nzb_cleanup_behavior:
    on_success: 'delete' # Options: delete, keep (default: delete)
    on_failure: 'delete' # Options: delete, keep (default: delete)

[javi11]

Remove the log file

[javi11]

resolve the conflicts

[drondeseries]

Done