fix(turnstile): route CAPTCHA verification through the configured proxy (#38412) (#38420)

Backport #38412

Fixes #38217

Co-authored-by: Lovepreet Singh <lovepreet.singh61182@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/util/sync.go

@@ -0,0 +1,51 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+type onceValueResult[T any] struct {
+	value T
+	panic any
+}
+
+// OnceValue is similar to Golang's "sync.OnceValue", but can be reset.
+type OnceValue[T any] struct {
+	Func func() T
+	mu   sync.Mutex
+	res  atomic.Pointer[onceValueResult[T]]
+}
+
+func (o *OnceValue[T]) Value() T {
+	res := o.res.Load()
+	if res == nil {
+		o.mu.Lock()
+		defer o.mu.Unlock()
+		res = o.res.Load()
+		if res == nil {
+			res = &onceValueResult[T]{}
+			defer func() {
+				res.panic = recover()
+				o.res.Store(res)
+				if res.panic != nil {
+					panic(res.panic)
+				}
+			}()
+			res.value = o.Func()
+		}
+	}
+	if res.panic != nil {
+		panic(res.panic)
+	}
+	return res.value
+}
+
+func (o *OnceValue[T]) Reset() {
+	o.mu.Lock()
+	defer o.mu.Unlock()
+	o.res.Store(nil)
+}