feat(httpclient): add circuit breaker for CDN and RIPEstat

Per-host circuit breaker с retry для CDN fetch и RIPEstat; порог 5 ошибок,
cooldown 30s.

Co-authored-by: Cursor <[email protected]>

internal/httpclient/circuit_test.go

@@ -0,0 +1,34 @@
+package httpclient
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestDoWithBreaker_opensAfterFailures(t *testing.T) {
+	ResetHostBreakers()
+	var calls atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls.Add(1)
+		http.Error(w, "fail", http.StatusBadGateway)
+	}))
+	defer srv.Close()
+
+	hc := New(5 * time.Second)
+	for i := 0; i < defaultBreakerThreshold*3; i++ {
+		req, _ := http.NewRequest(http.MethodGet, srv.URL, nil)
+		_, _ = DoWithBreaker(context.Background(), hc, req, 1)
+	}
+	req, _ := http.NewRequest(http.MethodGet, srv.URL, nil)
+	_, err := DoWithBreaker(context.Background(), hc, req, 1)
+	if err == nil || err.Error() == "" {
+		t.Fatal("expected circuit open error")
+	}
+	if got := calls.Load(); got == 0 {
+		t.Fatal("expected at least one upstream call")
+	}
+}