feat(import): resolve color variety ID mappings mismatch, support unnamed parents in virtual litters, and implement SaleContract updates

GerbilManagerWebAPI/Models/Litter.cs

@@ -8,7 +8,7 @@ namespace GerbilManagerWebAPI.Models
         [Key]
         public Guid Id { get; set; }
         public required string Name { get; set; }
-        public DateOnly Date { get; set; }
+        public DateOnly? Date { get; set; }
 
         /// <summary>Total born count (was "Strength").</summary>
         public int? TotalBorn { get; set; }