BE - Implement passkey model layer

Context

Extend webauthn_registrations table to support passkeys.

Implementation Plan

• Add migration for authentication_mode column
• Add last_used column for the account_table
• Add passkey_eligible column to prep for the future upgrade flow
• Update WebauthnRegistration model (enums and validations)
• Update User model (associations & validations)
  • enum :authentication_mode, %i[passwordless second_factor].index_with(&:to_s)
• Add specs