feat(domain): add domain services and events

- MessageIdGenerator: Generates unique message IDs using legacy-compatible SHA256 hash
  Algorithm matches VB.NET system for duplicate detection
- EmailProcessedEvent: MediatR domain event for email processing completion

Domain services encapsulate business logic that doesn't belong to entities.

src/DigitalData.EmailProfiler.Domain/Events/EmailProcessedEvent.cs

@@ -0,0 +1,22 @@
+using MediatR;
+using DigitalData.EmailProfiler.Domain.Enums;
+
+namespace DigitalData.EmailProfiler.Domain.Events;
+
+public class EmailProcessedEvent : INotification
+{
+    public int EmailHistoryId { get; }
+    public int ProfileId { get; }
+    public string MessageId { get; }
+    public EmailStatus Status { get; }
+    public DateTime ProcessedDate { get; }
+
+    public EmailProcessedEvent(int emailHistoryId, int profileId, string messageId, EmailStatus status)
+    {
+        EmailHistoryId = emailHistoryId;
+        ProfileId = profileId;
+        MessageId = messageId;
+        Status = status;
+        ProcessedDate = DateTime.UtcNow;
+    }
+}