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/handler/dto/settings.go

@@ -97,6 +97,7 @@ type SystemSettings struct {
 	// Gateway forwarding behavior
 	EnableFingerprintUnification bool `json:"enable_fingerprint_unification"`
 	EnableMetadataPassthrough    bool `json:"enable_metadata_passthrough"`
+	EnableCCHSigning             bool `json:"enable_cch_signing"`
 }
 
 type DefaultSubscriptionSetting struct {