Fix catalog item repricing on oven-only quote edits

QuoteItem was missing IncludePrepCost, so the Edit GET always deserialized
it as true (DTO default). On save, prep service labor was added on top of
the catalog base price, silently bumping prices whenever any quote field
(e.g. oven cycle minutes) was changed without touching items.

Migration defaults new column to false for catalog items and true for
non-catalog items (matching the wizard's historical defaults).

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

src/PowderCoating.Core/Entities/QuoteItem.cs

@@ -32,6 +32,11 @@ public class QuoteItem : BaseEntity
     public bool RequiresSandblasting { get; set; }
     public bool RequiresMasking { get; set; }
     public int EstimatedMinutes { get; set; }
+
+    // Whether to add prep service labor cost on top of this item's base price.
+    // Defaults to false for catalog items (catalog price assumed to include standard labor);
+    // true for calculated items where prep is a separate billable add-on.
+    public bool IncludePrepCost { get; set; }
     
     public string? Notes { get; set; }