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.Core/Migrations/FabWorksDbContextModelSnapshot.cs

@@ -102,6 +102,9 @@ namespace FabWorks.Core.Migrations
                     b.Property<double?>("KFactor")
                         .HasColumnType("float");
 
+                    b.Property<int>("Revision")
+                        .HasColumnType("int");
+
                     b.Property<double?>("Thickness")
                         .HasColumnType("float");