fix(domain): use DateTime.Now instead of DateTime.UtcNow for legacy compatibility

CRITICAL FIX: Replace all DateTime.UtcNow with DateTime.Now throughout the application.

Reason: Legacy VB.NET system uses local server time, and database stores all
timestamps as local time. Using UTC breaks compatibility and causes incorrect
time comparisons.

Changes:
- EmailProcessedEvent: ProcessedDate now uses DateTime.Now
- EmailHistory.MarkAsProcessed(): ProcessedDate now uses DateTime.Now
- EmailHistory.MarkAsFailed(): ProcessedDate now uses DateTime.Now
- EmailProfile.UpdateLastPollTime(): LastPollTime now uses DateTime.Now
- EmailProfile.ShouldPoll(): Poll interval comparison now uses DateTime.Now

Documentation:
- Added critical note to agents.md about DateTime usage
- Includes examples and detailed explanation for future developers

This ensures all date/time operations remain compatible with legacy database.

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

@@ -3,20 +3,11 @@ using DigitalData.EmailProfiler.Domain.Enums;
 
 namespace DigitalData.EmailProfiler.Domain.Events;
 
-public class EmailProcessedEvent : INotification
+public class EmailProcessedEvent(int emailHistoryId, int profileId, string messageId, EmailStatus status) : 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;
-    }
+    public int EmailHistoryId { get; } = emailHistoryId;
+    public int ProfileId { get; } = profileId;
+    public string MessageId { get; } = messageId;
+    public EmailStatus Status { get; } = status;
+    public DateTime ProcessedDate { get; } = DateTime.Now;
 }