feat: implement notification system with repository, service, and controller layers

Distribution/Features/Notifications/NotificationDTOs.cs

@@ -0,0 +1,28 @@
+using Distribution.Models;
+
+namespace Distribution.Features.Notifications;
+
+/// <param name="Values">Object whose public properties fill the {Token} placeholders in the type's templates.</param>
+/// <param name="CreatedByUserId">The acting user; excluded from the recipients.</param>
+public record NotificationArgs(
+    object? Values,
+    string EntityType,
+    Guid EntityId,
+    string Link,
+    Guid? CreatedByUserId = null);
+
+/// <param name="Id">The caller's own recipient row id — use it for read / delete.</param>
+public record NotificationResponse(
+    Guid Id,
+    string Type,
+    NotificationSeverity Severity,
+    string Title,
+    string Message,
+    string EntityType,
+    Guid EntityId,
+    string Link,
+    DateTime CreatedAt,
+    bool IsRead,
+    DateTime? ReadAt);
+
+public record UnreadCountResponse(int Count);