feat(DbExporter): create project structure and definition model

Add DbExporter tool project with:
- Project file targeting .NET 10 with required packages
  (protobuf-net-data, ZstdSharp.Port, Microsoft.Data.SqlClient,
  Oracle.ManagedDataAccess.Core)
- ExportDefinition model for JSON-based export configuration
- Placeholder Program.cs entry point

Tools/DbExporter/Program.cs

@@ -0,0 +1,9 @@
+namespace DbExporter;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        Console.WriteLine("DbExporter - Database Export Tool");
+    }
+}