feat: adapt Flatline to new OIDC-based principal verification

[uoemai]

This PR complements mollyim/flatline-whisper-service#16 and mollyim/flatline-whisper-service#17.

It adapts the infrastructure to work with OIDC-based principal verification.

More specifically, this PR implements the following:

• Deploys and configures Authelia as the default verification provider for the Flatline prototype.
• Updates the Whisper configuration to include the new tables added in the aforementioned PRs.
• Executes all tests for Whisper, which have been updated with the changes in the aforementioned PRs.
• Removes the unnecessary Registration Service component, now covered by Whisper + Authelia.
• Removes the unnecessary Contact Discovery Service component, now incompatible with principals.
• Updates CI workflows and documentation to remove the Registration Service component.
• Updates architecture documentation to include the Authelia component.
• Documents the new verification/registration process for Flatline.