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; }

GerbilManagerWebAPI/Models/SaleContract.cs

@@ -41,5 +41,9 @@ namespace GerbilManagerWebAPI.Models
         public Guid SaleContractId { get; set; }
         public Guid GerbilId { get; set; }
         public Gerbil? Gerbil { get; set; }
+
+        /// <summary>Optional gewähltes Tierfoto (<see cref="GerbilPhoto"/>) fürs Vertragsbild.
+        /// Nur die Id wird gespeichert; beim PDF-Rendern wird die Datei nachgeladen.</summary>
+        public Guid? PhotoId { get; set; }
     }
 }