Complete the API token export/import feature with proper version
handling and backward compatibility:
- Bump export format to version 4.1 to indicate API token support
- Import API tokens when loading v4.1 exports
- Handle version compatibility gracefully:
- v4.1: Full support including API tokens
- v4.0: Notice that tokens weren't included (backward compatible)
- Other: Warning but best-effort import
- Initialize empty array instead of nil for cleaner JSON
This ensures API tokens are properly preserved when migrating or
restoring Pulse instances while maintaining backward compatibility
with older exports.
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