Hybrid digital signatures for iOS devices

Currently, the solution is designed to fully use post-quantum cryptography (PQC), supported only on iOS 26+. Ideally, the solution should fallback to Elliptic Curve cryptography for older iOS models. Adjustments would be needed on the mobile app iOS implementation and on the upstream API signature checks.

## Potential design approaches

1. During the device enrollment process, having a metadata attribute describing which crypto scheme was used
2. Upstream API transparently attempting checks with try-catch fallback

Review the mobile app iOS implementation and the upstream API signature checks first. Compare enrollment metadata identifying the crypto scheme with transparent verification fallback, and define completion as older iOS devices using Elliptic Curve signatures while the API accepts both schemes.

Written by the indexing model from the issue text.