feat(federation): surface sourceMessageId/sourceInstance in DM message payloads

Both REST and WebSocket DM message payloads now include the relay
identity fields. This enables client-side deduplication of relay copies.

packages/server/src/routes/dm.ts

@@ -80,6 +80,8 @@ export function buildDmMessageWithUser(
     content: message.content,
     editedAt: message.editedAt,
     createdAt: message.createdAt,
+    sourceMessageId: message.sourceMessageId ?? null,
+    sourceInstance: message.sourceInstance ?? null,
     user: sanitizeUser(user),
     attachments: attachmentRows.map(a => ({
       id: a.id,