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/Common/Interfaces/IImapEmailService.cs

@@ -13,10 +13,15 @@ public interface IImapEmailService
     /// </summary>
     /// <param name="account">Account whose IMAP settings will be used.</param>
     /// <param name="filter">Filter to apply when fetching emails.</param>
+    /// <param name="markAsSeen">
+    /// 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 read (uses <c>BODY.PEEK</c> internally).
+    /// </param>
     /// <param name="cancellationToken">Cancellation token.</param>
     Task<IEnumerable<ReceivedEmailContext>> FetchEmailsAsync(
         EmailAccountDto account,
         MailSearchFilter filter,
+        bool markAsSeen = true,
         CancellationToken cancellationToken = default);
 
     /// <summary>
@@ -30,11 +35,16 @@ public interface IImapEmailService
     /// <summary>
     /// Fetches a single email by its UID.
     /// </summary>
+    /// <param name="markAsSeen">
+    /// When <see langword="true"/> (default), the message is marked as <c>\Seen</c> on the server.
+    /// Set to <see langword="false"/> for a non-destructive read.
+    /// </param>
     Task<ReceivedEmailContext?> FetchEmailByUidAsync(
         EmailAccountDto account,
         long uid,
         string folder = "INBOX",
         bool withAttachments = false,
+        bool markAsSeen = true,
         CancellationToken cancellationToken = default);
 
     /// <summary>