[Cells 1.0] (Size: S) Decide what to do with `secret_key_base`

The secret_key_base is used as the input secret to the application’s key generator, which in turn is used to create all ActiveSupport::MessageVerifier and ActiveSupport::MessageEncryptor instances, including the ones that sign and encrypt cookies.

Does it need to be the same for all cells ?

Proposal (2024-09-19)

secret_key_base is used for: encrypted cookies, signed cookies, and Active Storage files, based on https://guides.rubyonrails.org/security.html#session-storage. We don't use Active Storage, as far as I know, so it's only cookies that are affected.

Given that for Cells 1.0, users will be tied to a single cell, and since the session cookie will include the cell ID in its name, I think it's fine to have a unique secret_key_base per Cell then.