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/Entities/EmailProfile.cs

@@ -77,12 +77,12 @@ public class EmailProfile : BaseEntity, IAggregateRoot
     public virtual ICollection<EmailHistory> EmailHistories { get; set; } = new List<EmailHistory>();
 
     // Domain methods
-    public void UpdateLastPollTime() => LastPollTime = DateTime.UtcNow;
+    public void UpdateLastPollTime() => LastPollTime = DateTime.Now;
 
     public bool ShouldPoll()
     {
         if (!IsActive) return false;
         if (!LastPollTime.HasValue) return true;
-        return DateTime.UtcNow >= LastPollTime.Value.AddMinutes(PollIntervalMinutes);
+        return DateTime.Now >= LastPollTime.Value.AddMinutes(PollIntervalMinutes);
     }
 }