feat(e2ee): tell the user when their device's key uploads are stuck (#201 KE-1)

When the server keeps rejecting this device's /keys/upload with 400 "One
time key ... already exists", the device's encryption store has diverged
and the SDK retries forever (matrix-rust-sdk#5200, still open). Synapse
saw 474k of these from May 30 to Jul 8 2026. The fix is a fresh login, so:

- keyUploadWatch observes /keys/upload responses through the client's
  fetchFn (the response the SDK gets is untouched; a clone is read).
  Three conflicts within 15 min = stuck; any successful upload clears it.
- KeyUploadBanner (critical strip, same slot as the sync/clock banners):
  "Encryption on this device is out of sync" with Details and
  "Fix: sign out…", which opens the normal logout dialog (it already
  warns when the device isn't verified / backed up).

Server side, Grafana now alerts on the same signal (E2EE Key Upload
Conflicts) and on delayed-event 5xx (Call Delayed-Event Errors).

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

src/app/utils/keyUploadWatch.test.ts

@@ -0,0 +1,86 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  getKeyUploadWatch,
+  isKeyUploadUrl,
+  isOtkConflict,
+  recordKeyUpload,
+  resetKeyUploadWatch,
+  STUCK_AFTER,
+  subscribeKeyUploadWatch,
+  WINDOW_MS,
+  watchKeyUploads,
+} from './keyUploadWatch';
+
+const CONFLICT = JSON.stringify({
+  errcode: 'M_UNKNOWN',
+  error:
+    'One time key signed_curve25519:AAAAAAAAAA0 already exists. Old key: {...}; new key: {...}',
+});
+
+test('[#201] recognises the upload endpoint and the conflict body', () => {
+  assert.equal(isKeyUploadUrl('https://hs.example/_matrix/client/v3/keys/upload'), true);
+  assert.equal(isKeyUploadUrl('https://hs.example/_matrix/client/v3/keys/upload/'), true);
+  assert.equal(isKeyUploadUrl('https://hs.example/_matrix/client/v3/keys/query'), false);
+  assert.equal(isKeyUploadUrl('https://hs.example/_matrix/client/v3/room_keys/keys'), false);
+  assert.equal(isOtkConflict(400, CONFLICT), true);
+  assert.equal(isOtkConflict(400, '{"errcode":"M_BAD_JSON"}'), false);
+  assert.equal(isOtkConflict(500, CONFLICT), false);
+});
+
+test('[#201] stuck only after repeated conflicts inside the window', () => {
+  resetKeyUploadWatch();
+  const t0 = 1_000_000;
+  for (let i = 0; i < STUCK_AFTER - 1; i += 1) recordKeyUpload(400, CONFLICT, t0 + i * 1000);
+  assert.equal(getKeyUploadWatch().stuck, false, 'a couple of races is not divergence');
+  recordKeyUpload(400, CONFLICT, t0 + 5000);
+  assert.equal(getKeyUploadWatch().stuck, true);
+  assert.match(getKeyUploadWatch().lastError ?? '', /already exists/);
+});
+
+test('[#201] old conflicts age out; other 400s are ignored', () => {
+  resetKeyUploadWatch();
+  recordKeyUpload(400, CONFLICT, 0);
+  recordKeyUpload(400, CONFLICT, 1000);
+  recordKeyUpload(400, CONFLICT, WINDOW_MS + 60_000);
+  assert.equal(getKeyUploadWatch().stuck, false);
+  recordKeyUpload(400, '{"errcode":"M_BAD_JSON"}', WINDOW_MS + 61_000);
+  assert.equal(getKeyUploadWatch().conflicts, 1);
+});
+
+test('[#201] a successful upload clears it and notifies', () => {
+  resetKeyUploadWatch();
+  let calls = 0;
+  const off = subscribeKeyUploadWatch(() => {
+    calls += 1;
+  });
+  for (let i = 0; i < STUCK_AFTER; i += 1) recordKeyUpload(400, CONFLICT, i);
+  assert.equal(getKeyUploadWatch().stuck, true);
+  recordKeyUpload(200, '');
+  assert.equal(getKeyUploadWatch().stuck, false);
+  assert.ok(calls >= 2);
+  off();
+});
+
+test('[#201] the fetch wrapper observes without changing the response', async () => {
+  resetKeyUploadWatch();
+  const fake: typeof fetch = async (input) =>
+    String(input).endsWith('/keys/upload')
+      ? new Response(CONFLICT, { status: 400 })
+      : new Response('{}', { status: 200 });
+  const f = watchKeyUploads(fake);
+  const responses = await Promise.all(
+    Array.from({ length: STUCK_AFTER }, () =>
+      f('https://hs.example/_matrix/client/v3/keys/upload', { method: 'POST' }),
+    ),
+  );
+  const bodies = await Promise.all(responses.map((res) => res.text()));
+  responses.forEach((res) => assert.equal(res.status, 400));
+  bodies.forEach((b) => assert.match(b, /already exists/, 'the SDK can still read the body'));
+  await new Promise((r) => {
+    setTimeout(r, 10);
+  });
+  assert.equal(getKeyUploadWatch().stuck, true);
+  await f('https://hs.example/_matrix/client/v3/sync');
+  assert.equal(getKeyUploadWatch().stuck, true, 'unrelated requests change nothing');
+});