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/20250330182300_Proposal-Modified.cs

@@ -0,0 +1,40 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace FoodsharingSiegen.Server.Migrations
+{
+    /// <inheritdoc />
+    public partial class ProposalModified : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<DateTime>(
+                name: "Modified",
+                table: "Prospects",
+                type: "TEXT",
+                nullable: true);
+            
+            // Fill Modified column of existing rows
+            migrationBuilder.Sql(@"
+                UPDATE Prospects
+                SET Modified = COALESCE(
+                    (SELECT MAX(Created) 
+                     FROM Interactions 
+                     WHERE Interactions.ProspectID = Prospects.Id),
+                    Prospects.Created
+                );
+            ");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "Modified",
+                table: "Prospects");
+        }
+    }
+}