feat: add local database and file export infrastructure

Add EF Core DbContext with ExportRecord and BomItem entities for
tracking export history locally. Add FileExportService for saving
DXF and PDF files to a configurable output folder.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ExportDXF/Models/ExportRecord.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+
+namespace ExportDXF.Models
+{
+    public class ExportRecord
+    {
+        public int Id { get; set; }
+        public string DrawingNumber { get; set; }
+        public string SourceFilePath { get; set; }
+        public string OutputFolder { get; set; }
+        public DateTime ExportedAt { get; set; }
+        public string ExportedBy { get; set; }
+
+        public virtual ICollection<BomItem> BomItems { get; set; } = new List<BomItem>();
+    }
+}