feat: complete remaining Go backend modules

backend/internal/assets/remote_policy.go

@@ -0,0 +1,28 @@
+package assets
+
+import (
+	"net/http"
+	"time"
+
+	"git.nianxx.cn/wangxuming/NianAIGC/backend/internal/webhook"
+)
+
+// PublicDestinationPolicy reuses the backend's public-network validation and
+// DNS-pinned dialing policy for provider asset downloads.
+type PublicDestinationPolicy = webhook.PublicDestinationValidator
+
+func NewPublicDestinationPolicy(resolver webhook.IPResolver, dialer webhook.ContextDialer) *PublicDestinationPolicy {
+	return webhook.NewPublicDestinationPolicy(resolver, dialer)
+}
+
+// NewPublicHTTPRemoteFetcher is the production construction seam. Its
+// transport resolves and validates immediately before dialing the selected IP.
+func NewPublicHTTPRemoteFetcher(timeout time.Duration, maxBytes int64, policy *PublicDestinationPolicy) (*HTTPRemoteFetcher, error) {
+	if policy == nil {
+		return nil, errRemotePolicyRequired
+	}
+	transport := http.DefaultTransport.(*http.Transport).Clone()
+	transport.Proxy = nil
+	transport.DialContext = policy.DialContext
+	return NewPolicyHTTPRemoteFetcher(&http.Client{Timeout: timeout, Transport: transport}, maxBytes, policy)
+}