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

GerbilManagerWebAPI/Migrations/ApplicationContextModelSnapshot.cs

@@ -970,7 +970,7 @@ namespace GerbilManagerWebAPI.Migrations
                         .ValueGeneratedOnAdd()
                         .HasColumnType("uuid");
 
-                    b.Property<DateOnly>("Date")
+                    b.Property<DateOnly?>("Date")
                         .HasColumnType("date");
 
                     b.Property<int?>("DeathsWithin8Weeks")
@@ -1271,6 +1271,9 @@ namespace GerbilManagerWebAPI.Migrations
                     b.Property<Guid>("GerbilId")
                         .HasColumnType("uuid");
 
+                    b.Property<Guid?>("PhotoId")
+                        .HasColumnType("uuid");
+
                     b.HasKey("SaleContractId", "GerbilId");
 
                     b.HasIndex("GerbilId");