Introduced NotificationService for managing notification logic and recipient resolution.

- Integrated SignalR for real-time notification delivery to users.

Distribution/Models/Notification.cs

@@ -0,0 +1,33 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Distribution.Models
+{
+    public enum NotificationSeverity { Info, Success, Warning, Critical }
+
+    /// <summary>One row per event. Who it was delivered to lives in <see cref="NotificationRecipient"/>.</summary>
+    public class Notification : Template
+    {
+        [MaxLength(60)]
+        public string Type { get; set; } = string.Empty;
+
+        public NotificationSeverity Severity { get; set; }
+
+        [MaxLength(150)]
+        public string Title { get; set; } = string.Empty;
+
+        [MaxLength(500)]
+        public string Message { get; set; } = string.Empty;
+
+        [MaxLength(50)]
+        public string EntityType { get; set; } = string.Empty;
+
+        public Guid EntityId { get; set; }
+
+        [MaxLength(200)]
+        public string Link { get; set; } = string.Empty;
+
+        public Guid? CreatedByUserId { get; set; }
+
+        public ICollection<NotificationRecipient> Recipients { get; set; } = new List<NotificationRecipient>();
+    }
+}

Distribution/Models/NotificationRecipient.cs

@@ -0,0 +1,19 @@
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Distribution.Models
+{
+    /// <summary>One row per user per notification, so read state is per user.</summary>
+    public class NotificationRecipient : Template
+    {
+        public Guid FK_NotificationId { get; set; }
+        [ForeignKey("FK_NotificationId")]
+        public Notification Notification { get; set; }
+
+        public Guid FK_UserId { get; set; }
+        [ForeignKey("FK_UserId")]
+        public User User { get; set; }
+
+        public bool IsRead { get; set; }
+        public DateTime? ReadAt { get; set; }
+    }
+}