refactor: move go-chi/captcha into Gitea (#39529)

The `gitea.com/go-chi/captcha` package only exists for Gitea,
so it moves into `modules/imagecaptcha`.

- Reloading an expired challenge shows a new image instead of a broken one
- Every answer is consumed on its first check
- OpenID registration stops after a failed captcha

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/imagecaptcha/imagecaptcha.go

@@ -0,0 +1,122 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package imagecaptcha
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"image/color"
+	"image/png"
+	"math/rand/v2"
+	"net/http"
+	"regexp"
+	"strconv"
+	"sync"
+
+	"gitea.dev/modules/cache"
+	"gitea.dev/modules/log"
+	"gitea.dev/modules/util"
+)
+
+const (
+	cacheKeyPrefix = "captcha_"
+	ttlSeconds     = 600
+	codeLength     = 6
+)
+
+var primaryColors = []color.RGBA{ // readable on both light and dark backgrounds
+	{R: 234, G: 67, B: 53, A: 255},
+	{R: 66, G: 133, B: 244, A: 255},
+	{R: 52, G: 168, B: 83, A: 255},
+	{R: 251, G: 188, B: 5, A: 255},
+	{R: 171, G: 71, B: 188, A: 255},
+}
+
+type pngBufferPool struct{ sync.Pool }
+
+func (p *pngBufferPool) Get() *png.EncoderBuffer {
+	buf, _ := p.Pool.Get().(*png.EncoderBuffer)
+	return buf
+}
+
+func (p *pngBufferPool) Put(buf *png.EncoderBuffer) {
+	p.Pool.Put(buf)
+}
+
+func randomCode() string {
+	s := "000000" + strconv.Itoa(util.FastCryptoRandomInt(1000000))
+	return s[len(s)-6:]
+}
+
+var globalVars = sync.OnceValue(func() (ret struct {
+	IdLength int
+	IdRegexp *regexp.Regexp
+	NoiseKey []byte
+},
+) {
+	ret.IdLength = 40
+	ret.IdRegexp = regexp.MustCompile(`^[0-9a-f]{40}$`)
+	ret.NoiseKey = util.FastCryptoRandomBytes(32)
+	return
+})
+
+// noiseRand makes refetches of an image identical, so averaging them does not remove the noise
+func noiseRand(id, code string) *rand.Rand {
+	mac := hmac.New(sha256.New, globalVars().NoiseKey)
+	_, _ = mac.Write([]byte(id + "\x00" + code))
+	return util.FastCryptoRand([32]byte(mac.Sum(nil)))
+}
+
+func CreateNew() (string, error) {
+	id := util.FastCryptoRandomHex(globalVars().IdLength)
+	_, err := PrepareCode(id, true)
+	return id, err
+}
+
+func PrepareCode(id string, generateNew bool) (code string, err error) {
+	if !globalVars().IdRegexp.MatchString(id) {
+		return "", nil
+	}
+	cacheKey := cacheKeyPrefix + id
+	if generateNew {
+		code = randomCode()
+		if err = cache.GetCache().Put(cacheKey, code, ttlSeconds); err != nil {
+			return "", err
+		}
+	} else {
+		code, _ = cache.GetCache().Get(cacheKey)
+	}
+	return code, nil
+}
+
+func Verify(id, answer string) bool {
+	if !globalVars().IdRegexp.MatchString(id) {
+		return false
+	}
+	key := cacheKeyPrefix + id
+	code, ok := cache.GetCache().Get(key)
+	_ = cache.GetCache().Delete(key)
+	return ok && answer == code
+}
+
+func ServeImage(resp http.ResponseWriter, req *http.Request) {
+	urlQuery := req.URL.Query()
+	id, reload := urlQuery.Get("id"), urlQuery.Get("reload") != ""
+	code, err := PrepareCode(id, reload)
+	if err != nil {
+		log.Error("Failed to prepare captcha code for id %s: %v", id, err)
+		http.Error(resp, "Failed to prepare captcha code", http.StatusInternalServerError)
+		return
+	} else if code == "" {
+		http.NotFound(resp, req)
+		return
+	}
+
+	resp.Header().Set("Cache-Control", "no-store")
+	resp.Header().Set("Content-Type", "image/png")
+	if req.Method == http.MethodGet {
+		pngEncoder := png.Encoder{BufferPool: &pngBufferPool{}}
+		_ = pngEncoder.Encode(resp, drawImage(noiseRand(id, code), code))
+	}
+}