[Hard] Implement cryptographic verification for CIBA (Client Initiated Backchannel Auth)

[keepsloading]

Implementation issue mapping to the CIBA (Client Initiated Backchannel Auth) epic for the Notebook repository.

[keepsloading]

Friendly reminder for SSoC26 contributors: When you submit a Pull Request for this issue, please remember to also raise a dummy PR in the Memact/Context repository that references your original PR. This is required for your contribution to be considered for points on the SSoC leaderboard!