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.

agents.md

@@ -33,7 +33,29 @@ The `MessageIdGenerator` in `Domain.Services` must use **exactly the same algori
 
 **Algorithm**: SHA256 hash of `{originalMessageId}|{sender}|{date:yyyyMMddHHmmss}|{subject}`
 
-### 3. No Commits Without Permission
+### 3. DateTime Usage - ALWAYS Use Local Time
+**CRITICAL**: Always use `DateTime.Now` instead of `DateTime.UtcNow` throughout the entire application.
+
+**Reason**: The legacy system uses local server time, and the database stores all timestamps as local time. Using UTC would break compatibility and cause incorrect time comparisons.
+
+**Examples**:
+```csharp
+// ✅ CORRECT
+profile.CreatedDate = DateTime.Now;
+var lastPoll = DateTime.Now.AddMinutes(-profile.PollIntervalMinutes);
+
+// ❌ WRONG - DO NOT USE
+profile.CreatedDate = DateTime.UtcNow;  // NEVER USE UTC
+var lastPoll = DateTime.UtcNow.AddMinutes(-profile.PollIntervalMinutes);  // NEVER USE UTC
+```
+
+**Important**: This applies to:
+- All entity audit fields (CreatedDate, ModifiedDate, LastPollDate, etc.)
+- All date comparisons in business logic
+- All timestamps in logs and error messages
+- All date parameters in queries
+
+### 4. No Commits Without Permission
 **NEVER** commit changes to git automatically. Always wait for explicit user instruction to commit.
 
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