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.Server/Migrations/FsContextModelSnapshot.cs

@@ -99,6 +99,9 @@ namespace FoodsharingSiegen.Server.Migrations
                     b.Property<string>("Memo")
                         .HasColumnType("TEXT");
 
+                    b.Property<DateTime?>("Modified")
+                        .HasColumnType("TEXT");
+
                     b.Property<string>("Name")
                         .IsRequired()
                         .HasColumnType("TEXT");