fix(broker): libsodium dynamic import — extract .default for bun

await import('libsodium-wrappers') returns the namespace object in
bun, not the sodium API. randombytes_buf et al. live on .default.
Without this, every topic_create on the deployed broker errored
with 'sodium.randombytes_buf is not a function' and the WS handler
silently dropped — CLI saw a 5s timeout.

Confirmed via broker docker logs:
  warn ws message error: sodium.randombytes_buf is not a function

Same destructure pattern as crypto.ts (which uses the synchronous
default import).

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

apps/broker/src/broker.ts

@@ -634,7 +634,7 @@ async function generateTopicKeyBundle(): Promise<{
   senderPubkey: Uint8Array;
   senderPubkeyHex: string;
 }> {
-  const sodium = await import("libsodium-wrappers");
+  const sodium = (await import("libsodium-wrappers")).default;
   await sodium.ready;
   const topicKey = sodium.randombytes_buf(32);
   const sender = sodium.crypto_box_keypair();
@@ -674,7 +674,7 @@ async function sealTopicKeyForMember(args: {
     .where(eq(memberTable.id, args.memberId));
   if (!member) return;
 
-  const sodium = await import("libsodium-wrappers");
+  const sodium = (await import("libsodium-wrappers")).default;
   await sodium.ready;
   let recipientX25519: Uint8Array;
   try {