fix: various security fixes (#38406)

Addresses a batch of privately reported security issues, grouped by
area:

- **SSRF** - migration PR-patch/asset fetches, OAuth2 avatar & OpenID
discovery, pull-mirror URL re-validation, and the outbound proxy path.
- **Access-token scope** - prevent scope escalation on token creation;
keep public-only tokens confined (feeds, packages, Actions listings,
star/watch lists, limited/private owners).
- **Access control / disclosure** - go-get default-branch leak, webhook
authorization-header leak, watch clearing on private transitions,
label/attachment scoping.
- **Denial of service** - input bounds for npm dist-tags, Debian control
files, Arch file lists, and SSH keys.

### 📌 Attention for site admins

Not breaking - existing configs keep working - but two changes are worth
a look:

- **New SSRF protection** Outbound requests (migrations, OAuth2 avatars,
OpenID discovery, pull mirrors, proxy path) are now validated against
the allow/block host lists. If your instance legitimately reaches
internal hosts, you may need to add them to
`[security].ALLOWED_HOST_LIST` (and the relevant `ALLOW_LOCALNETWORKS`
settings).
- **Deprecation** `[webhook].ALLOWED_HOST_LIST` is deprecated and will
be removed in a future release. Use `[security].ALLOWED_HOST_LIST`
instead; the old key still works for now.

---------

Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

services/webhook/deliver.go

@@ -308,20 +308,14 @@ func webhookProxy(allowList *hostmatcher.HostMatchList) func(req *http.Request)
 // Init starts the hooks delivery thread
 func Init() error {
 	timeout := time.Duration(setting.Webhook.DeliverTimeout) * time.Second
+	allowedHostMatcher := hostmatcher.ParseHostMatchList("security.ALLOWED_HOST_LIST", setting.Webhook.AllowedHostList)
 
-	allowedHostListValue := setting.Webhook.AllowedHostList
-	if allowedHostListValue == "" {
-		allowedHostListValue = hostmatcher.MatchBuiltinExternal
-	}
-	allowedHostMatcher := hostmatcher.ParseHostMatchList("webhook.ALLOWED_HOST_LIST", allowedHostListValue)
-
+	// NewHTTPTransport enforces the allow-list on direct connections; when webhookProxy routes a request
+	// through a configured proxy, restricting the proxied target is the proxy server's responsibility.
 	webhookHTTPClient = &http.Client{
 		Timeout: timeout,
-		Transport: &http.Transport{
-			TLSClientConfig: &tls.Config{InsecureSkipVerify: setting.Webhook.SkipTLSVerify},
-			Proxy:           webhookProxy(allowedHostMatcher),
-			DialContext:     hostmatcher.NewDialContext("webhook", allowedHostMatcher, nil, setting.Webhook.ProxyURLFixed),
-		},
+		Transport: hostmatcher.NewHTTPTransport("webhook", allowedHostMatcher, nil, webhookProxy(allowedHostMatcher), setting.Webhook.ProxyURLFixed,
+			&tls.Config{InsecureSkipVerify: setting.Webhook.SkipTLSVerify}),
 	}
 
 	hookQueue = queue.CreateUniqueQueue(graceful.GetManager().ShutdownContext(), "webhook_sender", handler)