3.3 Add the client-side SignalR message for received invitations #326

Parent epic: #309

Summary

Add a client message such as TrustedSessionInvitationReceived.

Requirements

• Include the technical invitation ID.
• Include the encrypted envelope.
• Include only non-sensitive metadata required by the client.
• Wire the message into the existing client communication layer.

Acceptance criteria

• The client can receive the message without decrypting it in the communication layer.
• Message fields match the server contract.