perf: cache leaderboard aggregation result for 5 minutes

[xpoes123]

The leaderboard endpoint ran two full-collection $unwind aggregations on every request. Since the leaderboard changes infrequently, this is wasteful. This PR adds a 5-minute in-process cache in database/account-info/leaderboard.js so repeated calls — including all limit variants — share a single computed result.

Problem

leaderboard() in database/account-info/leaderboard.js invokes two $unwind aggregation pipelines on every HTTP request with no caching. Under any moderate load the leaderboard endpoint becomes a repeated full-collection scan.

Changes

• Adds two module-level variables (cachedOverall, cachedAt) and a CACHE_TTL constant (5 minutes).
• Returns the cached slice early if the cache is still fresh; recomputes and repopulates on expiry.

Risk & testing

No new dependencies. The cache is module-scoped so it resets on server restart — this matches expected behavior. The 5-minute TTL is consistent with how infrequently leaderboard standings change. semistandard and node --check pass.