feat: introduce training module with new entities, migrations, API, and frontend application, and add article type and validity days.

src/backend/Zentral.Domain/Entities/Training/TrainingRecord.cs

@@ -0,0 +1,39 @@
+using System;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Zentral.Domain.Entities.Training;
+
+public enum TrainingStatus
+{
+    Valid,
+    Expiring,
+    Expired
+}
+
+public class TrainingRecord : BaseEntity
+{
+    public int ClienteContattoId { get; set; }
+    public ClienteContatto ClienteContatto { get; set; } = null!;
+
+    public int ArticoloId { get; set; }
+    public Articolo Articolo { get; set; } = null!;
+
+    public DateTime DataEsecuzione { get; set; }
+    public DateTime? DataScadenza { get; set; }
+    
+    public string? AttestatoUrl { get; set; }
+    public string? Note { get; set; }
+
+    [NotMapped]
+    public TrainingStatus Stato 
+    { 
+        get 
+        {
+            if (!DataScadenza.HasValue) return TrainingStatus.Valid; // Or unknown? Assuming valid if no expiration.
+            var days = (DataScadenza.Value - DateTime.Today).TotalDays;
+            if (days < 0) return TrainingStatus.Expired;
+            if (days <= 30) return TrainingStatus.Expiring;
+            return TrainingStatus.Valid;
+        }
+    }
+}