chore(broker): wire mentions through WS topic_send + dedupe imports

WSSendMessage gains an optional mentions field; the broker forwards
it into appendTopicMessage so WS-driven topic sends get the same
write-time fan-out path as REST POST /v1/messages. v1 messages
(today's plaintext-base64) still fall back to a body regex when the
field is omitted, so existing CLIs aren't broken; v2 ciphertext
clients in phase 3 will populate it.

Also drops the duplicate meshMember import (kept the meshMember-as-
memberTable alias which the rest of the file uses).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/broker/src/index.ts

@@ -1952,6 +1952,7 @@ async function handleSend(
       senderSessionPubkey: conn.sessionPubkey ?? undefined,
       nonce: msg.nonce,
       ciphertext: msg.ciphertext,
+      mentions: msg.mentions,
     }).catch((e) =>
       log.warn("appendTopicMessage failed", { topic_id: topicId, err: String(e) }),
     );