feat: enforce realtime delegation audience on the message socket

The socket used to accept any valid token, so a full REST/actor token could open the
live stream. Now it can require a narrowly-scoped delegated token (aud=iios-message),
so a leaked socket token can't drive privileged REST, and vice-versa.

- MessagePrincipal gains `audience`, surfaced from both verify paths (OIDC aud, and the
  app-token `aud` claim).
- message.gateway: when IIOS_REALTIME_AUDIENCE is set, handleConnection accepts only a
  token whose aud matches (opt-in, like IIOS_REQUIRE_ATTESTATION; unset = no change).
- spec: verifier surfaces aud; gateway accepts iios-message, rejects iios-core / no-aud
  when enforcing, passes through when off.

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

packages/iios-service/src/messaging/message.gateway.ts

@@ -61,6 +61,14 @@ export class MessageGateway implements OnGatewayInit, OnGatewayConnection, OnGat
     try {
       const token = String(client.handshake.auth?.token ?? '');
       const principal = this.session.verify(token);
+      // Realtime delegation (least privilege): when IIOS_REALTIME_AUDIENCE is set, the socket
+      // accepts ONLY a token minted for that audience (a short-lived `iios-message` delegate) —
+      // a full REST/actor token (`iios-core`) or an un-scoped token cannot open the stream. So a
+      // leaked socket token can't drive privileged REST, and vice-versa. Unset → no enforcement.
+      const required = process.env.IIOS_REALTIME_AUDIENCE?.trim();
+      if (required && principal.audience !== required) {
+        throw new Error(`realtime audience "${principal.audience ?? '(none)'}" != required "${required}"`);
+      }
       client.data = { principal } satisfies SocketState;
     } catch (err) {
       this.logger.warn(`rejecting socket: ${(err as Error).message}`);