perf(internal/gzip): pool *gzip.Writer per middleware instance (#1654)

gzip.NewWriterLevel allocates fresh deflate window and hash table
buffers (~1.18 MiB) on every request. This commit pools them in a closure-local
sync.Pool so each middleware instance reuses its writers.

The level is validated once at setup (NewWriterLevel against
io.Discard); pooled writers are reset to io.Discard on Put so the
pool doesn't pin response writers between requests.

Only call site is RenderIndex (lib/http.go), which serves the
challenge page, so this directly cuts the per-challenge allocation
footprint.

I benchmarked the change using the following benchmark,
put in the commit message instead of in a file since it's pretty much useless
outside of this particular change.

```
package internal

import (
	"io"
	"net/http"
	"net/http/httptest"
	"testing"
)

func BenchmarkGzipMiddleware(b *testing.B) {
	payload := make([]byte, 4096)
	for i := range payload {
		payload[i] = byte(i)
	}

	inner := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Write(payload)
	})
	h := GzipMiddleware(1, inner)

	b.ReportAllocs()
	b.RunParallel(func(pb *testing.PB) {
		req := httptest.NewRequest(http.MethodGet, "/", nil)
		req.Header.Set("Accept-Encoding", "gzip")
		for pb.Next() {
			rec := httptest.NewRecorder()
			h.ServeHTTP(rec, req)
			io.Copy(io.Discard, rec.Body)
		}
	})
}
```

The results are pretty nice:

Benchmarks (Linux arm64, count=10, benchstat, vs origin/main):

  GzipMiddleware-8  sec/op     158.8µs ± 4%  ->   5.2µs ± 3%  -96.72% (p=0.000)
  GzipMiddleware-8  B/op       1180.6 KiB    ->   1.9 KiB     -99.84% (p=0.000)
  GzipMiddleware-8  allocs/op  32            ->  13           -59.38% (p=0.000)

Signed-off-by: jvoisin <julien.voisin@dustri.org>

docs/docs/CHANGELOG.md

@@ -39,6 +39,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix a race in the bbolt store where the asynchronous cleanup scheduled by an expired read could delete a value that had just been refreshed; the delete now only fires when the key still carries the same expired generation it observed.
 - Marginally increase the performances of requests processing
 - Marginally improve the performances of PoW validation
+- Significantly improve the performances of the gzip middleware
 
 ## v1.25.0: Necron