fix: make ReceivingDate non-nullable with migration initialization

- Change ReceivingDate from nullable to non-nullable DateTime
- Update migration to add column as nullable first
- Initialize existing entries with SamplingDate (or current date if null)
- Alter column to be non-nullable after initialization

Co-authored-by: markusrt <299753+markusrt@users.noreply.github.com>

Author: Copilot

NRZMyk.Services/Data/Entities/SentinelEntry.cs

@@ -12,7 +12,7 @@ public class SentinelEntry : BaseEntity, IAggregateRoot, ISentinelEntry
         public DateTime? SamplingDate { get; set; }
 
         [Display(Name = "Eingangsdatum")]
-        public DateTime? ReceivingDate { get; set; }
+        public DateTime ReceivingDate { get; set; }
 
         [Display(Name = "Labnr. Einsender")]
         public string SenderLaboratoryNumber { get; set; }

NRZMyk.Services/Data/Migrations/20260207061615_SentinelEntry_AddReceivingDate.cs

@@ -7,10 +7,22 @@ public partial class SentinelEntry_AddReceivingDate : Migration
     {
         protected override void Up(MigrationBuilder migrationBuilder)
         {
+            // Add column as nullable first
             migrationBuilder.AddColumn<DateTime>(
                 name: "ReceivingDate",
                 table: "SentinelEntries",
                 nullable: true);
+
+            // Initialize ReceivingDate with SamplingDate for existing entries
+            migrationBuilder.Sql(
+                @"UPDATE SentinelEntries 
+                  SET ReceivingDate = COALESCE(SamplingDate, GETDATE())");
+
+            // Make the column non-nullable
+            migrationBuilder.AlterColumn<DateTime>(
+                name: "ReceivingDate",
+                table: "SentinelEntries",
+                nullable: false);
         }
 
         protected override void Down(MigrationBuilder migrationBuilder)