feat: sync billing header cc_version with User-Agent and add opt-in CCH signing

- Sync cc_version in x-anthropic-billing-header with the fingerprint
  User-Agent version, preserving the message-derived suffix
- Implement xxHash64-based CCH signing to replace the cch=00000
  placeholder with a computed hash
- Add admin toggle (enable_cch_signing) under gateway forwarding settings,
  disabled by default

backend/internal/service/settings_view.go

@@ -78,6 +78,7 @@ type SystemSettings struct {
 	// Gateway forwarding behavior
 	EnableFingerprintUnification bool // 是否统一 OAuth 账号的指纹头（默认 true）
 	EnableMetadataPassthrough    bool // 是否透传客户端原始 metadata（默认 false）
+	EnableCCHSigning             bool // 是否对 billing header cch 进行签名（默认 false）
 }
 
 type DefaultSubscriptionSetting struct {