> Server actions are public, unauthenticated routes

Why can't they be authenticated? That seems like the obvious fix. Otherwise how you are handing out the correct customer_id unless you authenticate somehow?

This scheme also complicates API key rotation, although you can work around it by trying to decrypt with both the old and new key if you use e.g. authenticated encryption.

This also has no mechanism for expiration (besides API key rotation). If you add an expiration time and sign it, then you essentially created an authentication token that you use as the customer_id.