feat: implement training notification management and new training pages

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

@@ -0,0 +1,35 @@
+using System;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Zentral.Domain.Entities.Training;
+
+public enum NotificationStatus
+{
+    Pending,
+    Approved,
+    Sent,
+    Error
+}
+
+public class TrainingNotification : BaseEntity
+{
+    public int? ClienteId { get; set; } // Notifications are grouped by Client (Company)
+    public Cliente? Cliente { get; set; }
+
+    public string RecipientEmail { get; set; } = string.Empty;
+    public string Subject { get; set; } = string.Empty;
+    public string Body { get; set; } = string.Empty;
+
+    public DateTime ScheduledDate { get; set; }
+    public DateTime? SentDate { get; set; }
+    
+    // JSON array of TrainingRecord IDs included in this notification
+    public string IncludedRecordIds { get; set; } = "[]"; 
+
+    public NotificationStatus Status { get; set; } = NotificationStatus.Pending;
+    public string? ErrorMessage { get; set; }
+
+    // Optional: Link to specific TrainingRecords if needed for traceability, 
+    // but if it's a grouped email, maybe just a JSON list or text description in Body is enough.
+    // Let's keep it simple for now, the Body will contain the details.
+}