feat: add event loop block notifications and env flag (#2842)

Add colored console warnings when the event loop is blocked and wire
a feature flag to enable/disable notifications- Introduce
notifyEventLoopBlocked() in eventLoopMonitor.server.ts to
  log a colored warning with blocked and async type.
- Call notifyEventLoopBlocked() when an event-loop stall is detected.
- Add EVENT_LOOP_MONITOR_NOTIFY_ENABLED to env schema with a default of
  "0" so notifications are off by default.
- Will notify when over the `EVENT_LOOP_MONITOR_THRESHOLD_MS` env var

This makes it easier to spot long event-loop stalls during development
or when notifications are explicitly enabled.

<img width="840" height="132" alt="CleanShot 2026-01-07 at 15 03 24@2x"
src="https://github.com/user-attachments/assets/be20fa6a-be2b-46a1-aa89-d0913ed8b5b3"
/>

Author: ericallam

apps/webapp/app/env.server.ts

@@ -1268,6 +1268,7 @@ const EnvironmentSchema = z
     EVENT_LOOP_MONITOR_THRESHOLD_MS: z.coerce.number().int().default(100),
     EVENT_LOOP_MONITOR_UTILIZATION_INTERVAL_MS: z.coerce.number().int().default(1000),
     EVENT_LOOP_MONITOR_UTILIZATION_SAMPLE_RATE: z.coerce.number().default(0.05),
+    EVENT_LOOP_MONITOR_NOTIFY_ENABLED: z.string().default("0"),
 
     VERY_SLOW_QUERY_THRESHOLD_MS: z.coerce.number().int().optional(),
 

apps/webapp/app/eventLoopMonitor.server.ts

@@ -9,6 +9,23 @@ import { signalsEmitter } from "./services/signals.server";
 
 const THRESHOLD_NS = env.EVENT_LOOP_MONITOR_THRESHOLD_MS * 1e6;
 
+// ANSI color codes for terminal output
+const RED = "\x1b[31m";
+const YELLOW = "\x1b[33m";
+const RESET = "\x1b[0m";
+
+function notifyEventLoopBlocked(timeMs: number, asyncType: string): void {
+  if (env.EVENT_LOOP_MONITOR_NOTIFY_ENABLED !== "1") {
+    return;
+  }
+
+  console.warn(
+    `${RED}⚠️  Event loop blocked${RESET} for ${YELLOW}${timeMs.toFixed(
+      1
+    )}ms${RESET} (${asyncType})`
+  );
+}
+
 const cache = new Map<number, { type: string; start?: [number, number]; parentCtx?: Context }>();
 
 function init(asyncId: number, type: string, triggerAsyncId: number, resource: any) {
@@ -66,6 +83,8 @@ function after(asyncId: number) {
     );
 
     newSpan.end();
+
+    notifyEventLoopBlocked(time, cached.type);
   }
 }