Stream outbound-delivery diagnostics as Server-Sent Events

Opens a text/event-stream connection that streams each stage of an outbound delivery attempt to target (a domain or email address): MX lookup, MTA-STS fetch, TLSA/DANE validation, SMTP conversation, and so on. Each SSE frame carries a JSON-encoded DeliveryStage wrapped in a single-element array (data: [{...}]). The stream ends with a final completed event. Requires LiveDeliveryTest permission; may also be authenticated via the ?token= query parameter obtained from /api/token/delivery.