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/ExportDefinition.cs

@@ -0,0 +1,21 @@
+using System.Text.Json.Serialization;
+
+namespace DbExporter;
+
+public sealed class ExportDefinition
+{
+    [JsonPropertyName("providerType")]
+    public required string ProviderType { get; init; }
+
+    [JsonPropertyName("connectionString")]
+    public required string ConnectionString { get; init; }
+
+    [JsonPropertyName("query")]
+    public required string Query { get; init; }
+
+    [JsonPropertyName("outputPath")]
+    public required string OutputPath { get; init; }
+
+    [JsonPropertyName("compressionLevel")]
+    public int CompressionLevel { get; init; } = 10;
+}