Add 'Modified' tracking for Prospect entity

Introduce a 'Modified' field to track the last modification date of prospects. Automatically update this field on changes and integrate it in UI display for better transparency.

FoodsharingSiegen.Contracts/Entity/Prospect.cs

@@ -42,6 +42,11 @@ namespace FoodsharingSiegen.Contracts.Entity
         /// </summary>
         public string? Memo { get; set; }
 
+        /// <summary>
+        ///     Gets or sets the value indicating the last modification date and time.
+        /// </summary>
+        public DateTime? Modified { get; set; }
+
         /// <summary>
         ///     Gets or sets the value of the name (ab)
         /// </summary>