headless JWKS 워커 실패 backoff 및 timeout 단축

backend/internal/domain/headless_jwks_cache.go

@@ -17,6 +17,7 @@ type HeadlessJWKSCacheState struct {
 	CachedAt                     *time.Time              `json:"cachedAt,omitempty"`
 	ExpiresAt                    *time.Time              `json:"expiresAt,omitempty"`
 	LastCheckedAt                *time.Time              `json:"lastCheckedAt,omitempty"`
+	NextRetryAt                  *time.Time              `json:"nextRetryAt,omitempty"`
 	LastSuccessfulVerificationAt *time.Time              `json:"lastSuccessfulVerificationAt,omitempty"`
 	LastRefreshStatus            string                  `json:"lastRefreshStatus,omitempty"`
 	LastError                    string                  `json:"lastError,omitempty"`