feat(service): P2.2 MessageService — native send + receipts/unread + traceId

openThread (create-or-join), send (idempotent, bumps other participants' unread,
message.sent outbox event, traceId DB->event), markRead (READ receipt + unread
reset), contentRef attachment placeholder. Adds messageSent to contracts events.
5 DB-backed tests green; 24 total.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-contracts/src/events.ts

@@ -34,6 +34,7 @@ export interface CloudEvent<T = unknown> {
 export const IIOS_EVENTS = {
   rawReceived: 'com.insignia.iios.raw.received.v1',
   interactionNormalized: 'com.insignia.iios.interaction.normalized.v1',
+  messageSent: 'com.insignia.iios.message.sent.v1',
   messagePersisted: 'com.insignia.iios.message.persisted.v1',
   inboxItemCreated: 'com.insignia.iios.inbox.item.created.v1',
   ticketCreated: 'com.insignia.iios.support.ticket.created.v1',