fix(calls): one delayed-event restart in flight per id, with a timeout (#318)

During a homeserver stall Element Call retries update_delayed_event every
2 s, and the host opened a new HTTP request each time: 12 pending in a
60 s stall, all landing together on recovery (404 x5, 429 x7). Share the
in-flight restart per delay id and abort it after 5 s.

Refs #318

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/inFlight.test.ts

@@ -0,0 +1,32 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { dedupeInFlight } from './inFlight';
+
+test('dedupeInFlight: one request per key while pending, a new one after (#318)', async () => {
+  const once = dedupeInFlight<number>();
+  let calls = 0;
+  let release!: () => void;
+  const gate = new Promise<void>((r) => {
+    release = r;
+  });
+  const run = async () => {
+    calls += 1;
+    await gate;
+    return calls;
+  };
+  const a = once('d1', run);
+  const b = once('d1', run);
+  const c = once('d2', run);
+  assert.equal(a, b);
+  release();
+  assert.deepEqual(await Promise.all([a, b, c]), [2, 2, 2]);
+  assert.equal(calls, 2);
+  await once('d1', run);
+  assert.equal(calls, 3, 'settled requests are not reused');
+});
+
+test('dedupeInFlight: a rejection clears the key', async () => {
+  const once = dedupeInFlight<void>();
+  await assert.rejects(once('k', () => Promise.reject(new Error('boom'))));
+  await once('k', () => Promise.resolve());
+});