fix(goLive): gateway crash on screenshare stop — unhandledRejection during teardown

Test 00:32 confirmed the video pipeline WORKS (1410 frames @ 1280x720 sent,
ready=true, camera off) but the gateway crashed at stream stop:
unhandledRejection → graceful shutdown → systemd restart (bot offline).

Root cause candidates (both were fire-and-forget promises without .catch):
- BaseMediaConnection.setProtocols().then(...) — rejects when the PC is
  closed while setProtocols is in flight (stream teardown)
- void webRtcConn.createOffer().then(...) — rejects when the PC closes
  while the offer is still gathering

Fixes:
- .catch on both promise chains (log + continue; teardown is expected)
- unhandledRejection handler now treats transient stream errors (EPIPE,
  ERR_STREAM_DESTROYED, ERR_STREAM_WRITE_AFTER_END, ECONNRESET) like
  uncaughtException already does — warn + continue instead of shutting
  down the whole gateway. Non-transient rejections still log + shutdown
  (with String(reason) so the detail actually shows).

services/discord-gateway/src/app/bootstrap.ts

@@ -303,7 +303,27 @@ export async function initializeDiscordGateway() {
   });
 
   process.on("unhandledRejection", (reason, promise) => {
-    logger.error({ reason, promise }, "Unhandled rejection");
+    const err =
+      reason instanceof Error ? reason : new Error(String(reason ?? "unknown"));
+    const code = (err as NodeJS.ErrnoException).code ?? "";
+    // Same transient-teardown policy as uncaughtException: a rejection that
+    // fires while a stream is being torn down (EPIPE after ffmpeg stdin
+    // closes, write-after-destroy, socket reset) must NOT take the whole
+    // gateway offline. Log detail + continue. Everything else still shuts
+    // down so real bugs surface.
+    if (
+      code === "EPIPE" ||
+      code === "ERR_STREAM_DESTROYED" ||
+      code === "ERR_STREAM_WRITE_AFTER_END" ||
+      code === "ECONNRESET"
+    ) {
+      logger.warn(
+        { error: err },
+        "Unhandled rejection transient stream error — continuing",
+      );
+      return;
+    }
+    logger.error({ error: err, reason: String(reason) }, "Unhandled rejection");
     gracefulShutdown("unhandledRejection");
   });