Sender labels for API/MCP messages in Inbox

Currently, messages sent via the Public API or MCP (e.g., using send-reservation-message) do not show a "Sent by" label in the Hospitable inbox. This makes it difficult for teams to distinguish between messages sent by an AI assistant/integration and those sent by human teammates.

The user requests the ability to see a label for these messages (e.g., "Sent via API") or, ideally, an optional sender label field in the API endpoints that allows them to specify a name (e.g., "Elite Assistant") to be displayed in the inbox for internal tracking.