fix: resolve errcheck lint and add missing enable_cch_signing to test

- Suppress errcheck for xxhash Digest.Write (never returns error)
- Add enable_cch_signing field to settings API contract test

backend/internal/server/api_contract_test.go

@@ -536,6 +536,7 @@ func TestAPIContracts(t *testing.T) {
 					"max_claude_code_version": "",
 					"allow_ungrouped_key_scheduling": false,
 					"backend_mode_enabled": false,
+					"enable_cch_signing": false,
 					"enable_fingerprint_unification": true,
 					"enable_metadata_passthrough": false,
 					"custom_menu_items": [],