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

[singhlovepreet9]

Fixes #38217

Problem

Turnstile CAPTCHA verification uses http.DefaultClient, so the request to challenges.cloudflare.com bypasses Gitea's configured HTTP proxy — unlike other outbound HTTP clients such as the update checker (modules/updatechecker/update_checker.go) and migrations. In deployments where egress is only permitted through the configured proxy, verification fails.

Fix

Build the client with proxy.Proxy() as the transport proxy, mirroring the update checker:

func httpClient() *http.Client {
	return &http.Client{
		Transport: &http.Transport{
			Proxy: proxy.Proxy(),
		},
	}
}

The client is built per call (rather than a package-level var) because proxy.Proxy() reads setting.Proxy when invoked; building it at request time ensures it reflects the loaded settings. When no proxy is configured, behavior is unchanged (proxy.Proxy() returns a no-op / http.ProxyFromEnvironment).

[Copilot]

Pull request overview

This PR updates the Turnstile CAPTCHA server-side verification HTTP client to honor Gitea’s configured outbound proxy settings, addressing deployments where egress is only allowed via proxy.

Changes:

• Route Turnstile verification HTTP requests through proxy.Proxy() instead of http.DefaultClient.
• Introduce a resettable util.OnceValue[T] helper to lazily initialize and cache a value.
• Add unit tests validating the Turnstile HTTP client’s proxy behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
modules/util/sync.go	Adds a resettable, lazy “once value” helper used to cache initialized values.
modules/util/sync_test.go	Adds unit tests for util.OnceValue behavior (repeat calls, panic caching, reset).
modules/turnstile/turnstile.go	Switches Turnstile verification to use an HTTP client that applies Gitea proxy configuration.
modules/turnstile/turnstile_test.go	Adds a test ensuring the Turnstile HTTP client honors the configured proxy.

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