refinement phase 3

dev/contracts/domains/messaging.md

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+
+---
+
+## Refinement phase 3 additions (REQ-028)
+
+- **`TicketSummaryDto`** gains `lastMessageAt` (last non-internal activity) + `unreadCount` (the caller's unread
+  non-internal messages from others; 0 on the admin queue). Unread is computed against the participant's
+  `last_read_at`, **stamped when the participant fetches the user-facing thread**.
+- **`GET /tickets`** gains a `bookingId` query filter (jump to a booking's coordination ticket).
+- **`POST /tickets/{id}/messages`** accepts an optional `clientMessageId` — a retried send with the same key is
+  deduplicated (returns the original) and the key is echoed on `PostMessageResult`.
+- **Message author = role label, not a name** (confirmed intentional, privacy): the DTO carries `senderId`; the
+  client derives the author label from the participant role. No raw identity/name is exposed.