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.go

@@ -0,0 +1,55 @@
+package httpclient
+
+import (
+	"sync"
+	"time"
+)
+
+const (
+	defaultBreakerThreshold = 5
+	defaultBreakerCooldown  = 30 * time.Second
+)
+
+type hostBreaker struct {
+	mu        sync.Mutex
+	failures  int
+	openUntil time.Time
+}
+
+var hostBreakers sync.Map // string -> *hostBreaker
+
+func breakerForHost(host string) *hostBreaker {
+	if host == "" {
+		host = "_"
+	}
+	v, _ := hostBreakers.LoadOrStore(host, &hostBreaker{})
+	return v.(*hostBreaker)
+}
+
+func (b *hostBreaker) allow() bool {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return time.Now().After(b.openUntil)
+}
+
+func (b *hostBreaker) recordSuccess() {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.failures = 0
+	b.openUntil = time.Time{}
+}
+
+func (b *hostBreaker) recordFailure() {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.failures++
+	if b.failures >= defaultBreakerThreshold {
+		b.openUntil = time.Now().Add(defaultBreakerCooldown)
+		b.failures = 0
+	}
+}
+
+// ResetHostBreakers clears all circuit breakers (tests only).
+func ResetHostBreakers() {
+	hostBreakers = sync.Map{}
+}