Replaces fnv.New64a() with direct inline hash computation over
fasthttp's []byte slices, eliminating 1 allocation per cache key
computation and 1 []byte(":") allocation.
Copy the request before spawning the background goroutine. The
fasthttp.RequestCtx is recycled after the handler returns, so passing
it to a goroutine causes data corruption under high concurrency.
The caller now AcquireRequest+CopyTo before go(), and the goroutine
releases it. backgroundRefresh no longer accepts ctx directly.
