fix(web): silence MaxListenersExceededWarning on proxied rewrites

Next 16.3's external rewrite proxy attaches 9 `close` listeners to each
ServerResponse, and Sentry adds 2 more, crossing Node's default limit of
10 on every `/ingest/*` and `/monitoring` request. The listeners are
released with the response, so this is a false positive that produces
~100-200k log lines/day in prod.

Raise the limit to 20 on server responses only, via the
`http.server.request.start` diagnostics channel, so the warning stays
meaningful for every other emitter.

See vercel/next.js#97757

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: brendan-kellam

packages/web/src/instrumentation.ts

@@ -1,4 +1,5 @@
 import * as Sentry from '@sentry/nextjs';
+import type { ServerResponse } from 'node:http';
 
 export async function register() {
     if (process.env.NEXT_RUNTIME === 'nodejs') {
@@ -10,6 +11,21 @@ export async function register() {
     }
 
     if (process.env.NEXT_RUNTIME === 'nodejs') {
+        // Proxied rewrites (`/ingest/*` for PostHog, the `/monitoring` Sentry
+        // tunnel) legitimately attach 11 `close` listeners to each response: 9
+        // from Next's rewrite proxy plus 2 from Sentry. That crosses Node's
+        // default limit of 10 and logs a MaxListenersExceededWarning on every
+        // such request, even though the listeners are released with the
+        // response. The request start channel fires before Next handles the
+        // request, and scoping the raise to responses keeps the warning
+        // meaningful for every other emitter. This can be removed once Next
+        // trims its proxy listeners.
+        // @see: https://github.com/vercel/next.js/issues/97757
+        const { subscribe } = await import('node:diagnostics_channel');
+        subscribe('http.server.request.start', (message) => {
+            (message as { response?: ServerResponse }).response?.setMaxListeners(20);
+        });
+
         const { startMetricsServer } = await import('./metricsServer');
         startMetricsServer();