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This answer is a bit self-serving, but I'm the author of the Digital Download plugin. It allows you to control how and when assets are downloaded.
You can restrict downloads by a time period, maximum number of downloads, or tie them to a logged-in user.
1
There is indeed an assets/save-asset controller. You will find it in vendor/craftcms/cms/src/controllers/AssetsController.php, the actionSaveAsset() function.
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