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

GerbilManagerWebAPI/Import/ImportDocxService.cs

@@ -81,7 +81,8 @@ namespace GerbilManagerWebAPI.Import
                 .Select(l => new { l.Id, l.Date })
                 .ToListAsync();
             var littersByDayNumber = littersInDb
-                .GroupBy(l => l.Date.DayNumber)
+                .Where(l => l.Date.HasValue)
+                .GroupBy(l => l.Date!.Value.DayNumber)
                 .ToDictionary(g => g.Key, g => g.ToList());
 
             // normalize(name)+litterDob → Gerbil snapshot (main-import enrich path)