read only mode. ledger_data makes sure ledger is present first

handlers/LedgerData.cpp

@@ -47,6 +47,12 @@ doLedgerData(
         response["error"] = "Empty database";
         return response;
     }
+    auto ledger = backend.fetchLedgerBySequence(*ledgerSequence);
+    if (!ledger)
+    {
+        response["error"] = "Ledger not found";
+        return response;
+    }
 
     std::optional<ripple::uint256> cursor;
     if (request.contains("cursor"))

reporting/ReportingETL.cpp

@@ -645,5 +645,7 @@ ReportingETL::ReportingETL(
     flatMapBackend_->open();
     if (config.contains("start_sequence"))
         startSequence_ = config.at("start_sequence").as_int64();
+    if (config.contains("read_only"))
+        readOnly_ = config.at("read_only").as_bool();
 }