fix: ratelimit crash under concurrent requests

[vlnst]

The rate limiter can crash the whole process when one IP sends several requests at once (for example, a question page with many images loading in parallel).

Cause: the per-IP counter is updated with a separate Load and Store, so simultaneous requests can lose increments. The count then reaches 0 while some 1-minute timers are still pending. One timer deletes the entry, and the next one runs val.(int) on a nil value and panics. The panic happens in a timer goroutine, so gin.Recovery() doesn't catch it and the process exits.

Fix: the timer returns early if the entry is already gone.

Testing: sent bursts of 28 simultaneous requests from one IP and waited for the timers. Before the fix, the server crashed with interface conversion: interface {} is nil, not int. After the fix, it stayed up in every run.