Links expiring in under a second silently stop getting cached

A short-lived link (a flash-sale link that expires in a few minutes, a one-time invite link) is, by construction, exactly the kind of link that gets hit repeatedly in a short window right before it expires -- that's what "short-lived and shared" traffic looks like. This bug means that once such a link has under a second left before expiresAt, every single GET /:code for it silently stops populating the Redis cache: no error, no log line, just a cache-aside path that quietly falls back to a Postgres query on every request instead of a Redis hit. For a link getting hammered in its final seconds -- exactly when a flash-sale link gets the most traffic, as people rush before it dies -- this turns what should be near-zero-latency cache hits into a Postgres query storm right at the worst possible moment.

setCachedLink(shortCode, { longUrl, expiresAt }) in src/lib/cache.js computes how many whole seconds remain until expiresAt and uses that as the Redis EX TTL (clamped to the default TTL). When less than 1000ms remain, that computation now rounds DOWN to 0 -- and the function then takes its "already expired, don't bother caching" early-return path even though the link has NOT expired yet; it just has a fraction of a second left.

// From a Node REPL or script with DATABASE_URL / REDIS_URL set: const { setCachedLink, getCachedLink } = require('./src/lib/cache');

await setCachedLink('demoCode', { longUrl: 'https://example.com/almost-gone', expiresAt: new Date(Date.now() + 500), // 500ms from now -- not expired });

console.log(await getCachedLink('demoCode')); // Expected: { longUrl: 'https://example.com/almost-gone', expiresAt: ... } // Actual: null -- it was never cached at all

The comment directly above the secondsUntilExpiry calculation in src/lib/cache.js already describes, in detail, exactly the failure mode this bug reintroduces, and explains which rounding direction avoids it. The code and the comment above it currently disagree with each other: one rounding function keeps the guarantee the comment describes, the other breaks it. That mismatch is the whole bug -- the fix is a single function call, not new logic.

Find the secondsUntilExpiry line in setCachedLink (src/lib/cache.js). Compare what it currently does against what its own comment says it should do, and correct the one thing that's out of sync.