feat(imap): add markAsSeen option to fetch queries and interface

- IImapEmailService.FetchEmailsAsync / FetchEmailByUidAsync now accept
  a bool markAsSeen = true parameter
- FetchEmailsQuery and FetchEmailByUidQuery expose MarkAsSeen { init; } = true
- markAsSeen=true  uses BODY[]      (server sets \\Seen, legacy-compatible)
- markAsSeen=false uses BODY.PEEK[] (non-destructive read, flag untouched)

src/core/DigitalData.MessagingService.Application/EmailReceiving/Queries/FetchEmailsQuery.cs

@@ -21,6 +21,12 @@ public record FetchEmailsQuery : IRequest<IEnumerable<ReceivedEmailContext>>
     /// Mail query used to filter and limit the emails retrieved.
     /// </summary>
     public MailSearchFilter Mail { get; init; } = new();
+
+    /// <summary>
+    /// When <see langword="true"/> (default), fetched messages are marked as <c>\Seen</c> on the server.
+    /// Set to <see langword="false"/> for a non-destructive peek.
+    /// </summary>
+    public bool MarkAsSeen { get; init; } = true;
 }
 
 public class FetchEmailsQueryHandler(
@@ -40,6 +46,7 @@ public class FetchEmailsQueryHandler(
         return await ImapService.FetchEmailsAsync(
             account,
             request.Mail,
+            request.MarkAsSeen,
             cancellationToken);
     }
 }