fix(analytics): wrap GA4 fetch in waitUntil to prevent dropped events

[askov]

Summary

The fire-and-forget fetch() in trackEvent() (lib/analytics.ts) returns a dangling Promise that the Vercel serverless runtime cancels the moment the response is returned. With GA4_MEASUREMENT_ID / GA4_API_SECRET correctly set in production, events were still never reaching google-analytics.com because the POST was cut off mid-flight — silently, since GA4's /mp/collect returns 204 regardless of whether anything was actually sent.

This was introduced together with the analytics module in #57.

Fix

Wrap the fetch in waitUntil() from @vercel/functions so the runtime keeps the function instance alive until the GA4 request settles — without blocking the airdrop response.

import { waitUntil } from "@vercel/functions";

waitUntil(
  fetch(url, { method: "POST", ... }).catch((err) => {
    console.error(`[ANALYTICS] Failed to send event "${name}":`, err);
  }),
);

Why waitUntil and not await?

await is also a valid fix — the cost is ~50–100 ms added to a request that's already 1–3 seconds (Turnstile + Solana RPC + backend logging). The choice for waitUntil here is to preserve the doc-stated contract that trackEvent "never blocks the airdrop response," and to isolate the airdrop path from any GA4 outage / TLS hang.

waitUntil is runtime-agnostic and reads a context object Vercel injects on globalThis. It does not change the deploy type, the runtime (Node.js Fluid Compute), regions, or pricing. Outside Vercel (self-hosted) it silently no-ops, matching the prior fire-and-forget semantics.

Test plan

• npx vitest run — all 111 tests pass locally (no test changes required; existing tracking tests mock @/lib/analytics)
• Deploy to a preview / staging environment with GA4_MEASUREMENT_ID + GA4_API_SECRET set
• Trigger an airdrop and confirm airdrop_success appears in GA4 Realtime within ~30 sec
• Trigger a failure path (invalid wallet) and confirm airdrop_failed with reason param appears
• Optional: temporarily swap the endpoint to /debug/mp/collect once to validate payload shape (GA4 silently drops invalid events on the prod endpoint)

[vercel]

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