feat: add revision tracking to CutTemplate and scope BOM items to export record

Each export record now keeps a complete BOM snapshot instead of moving
BomItems between records. CutTemplate gains a Revision field that
auto-increments when the content hash changes across exports for the
same drawing+item, and stays the same when the geometry is unchanged.

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

FabWorks.Api/DTOs/ExportDetailDto.cs

@@ -39,6 +39,7 @@ namespace FabWorks.Api.DTOs
         public int Id { get; set; }
         public string DxfFilePath { get; set; }
         public string ContentHash { get; set; }
+        public int Revision { get; set; }
         public double? Thickness { get; set; }
         public double? KFactor { get; set; }
         public double? DefaultBendRadius { get; set; }