fix(mobile): actually throttle unread catch-up at startup

[CryptoJones]

Problem

_catchUpUnreadEvents built a List<Future> before awaiting it:

for (final channel in channels) {
  futures.add(_catchUpUnreadEventsForChannel(...));   // starts immediately
}

const batchSize = 5;
for (var i = 0; i < futures.length; i += batchSize) {
  await Future.wait(futures.sublist(i, min(i + batchSize, futures.length)));
}

Invoking an async function in Dart starts it right away, so every per-channel history REQ was dispatched during the loop that builds the list. The batch loop below only awaited work that was already in flight, which made batchSize = 5 a no-op for concurrency.

A member of N channels therefore issued N concurrent history requests at startup, on top of the per-channel live subscriptions, presence, and the section sync.

Impact

On a relay using the default WS admission budget (BUZZ_RATE_LIMIT_HUMAN_WS_EVENTS_PER_SEC = 10 over the fixed 5s WS_BURST_WINDOW_SECS), the overflow is rejected:

[ChannelsNotifier] unread catch-up failed for <id>: Exception: rate-limited: quota exceeded; retry in 1s
[ChannelSectionsManager] startup sync incomplete; retrying in 2000ms (attempt 1)

The failed catch-ups then retry, re-spending the same fixed window, so a single overflow becomes a sustained failure loop. On a 14-channel account the app launches, fails to populate, and retries indefinitely — which presents to the user as the app hanging or crashing on open.

The relay-side comment on WS_BURST_WINDOW_SECS notes the window was sized for desktop startup, which establishes fewer simultaneous subscriptions than mobile.

Fix

Collect thunks rather than futures and invoke each batch as it is awaited, so at most batchSize requests are ever in flight. No behavioural change beyond concurrency.

Testing

• dart format — clean
• flutter analyze — no issues
• flutter test — 905 pass. One pre-existing failure in channel_detail_page_test.dart ("keeps follow mode off while a tall newest message stays visible") which reproduces identically on a clean origin/main checkout and is unrelated to this change.

Notes

This reduces the startup burst but does not eliminate it — the per-channel live subscriptions still fan out from a separate path. Filed #4440 for the relay side, where the WS admission budget always uses the human per-second limit even for agents and has no agent tier.

Proudly Made in Nebraska. Go Big Red! 🌽 https://xkcd.com/2347/