Remove MarkAsSeen endpoint from EmailController

The `MarkAsSeen` method in the `EmailController` class has been removed. This method provided an HTTP PATCH endpoint at the route `"seen"` to mark a single IMAP message as seen (read). It accepted a `MarkEmailAsSeenCommand` and a `CancellationToken`, used `mediator.Send` to process the command, and returned an HTTP 204 No Content response upon success.

The removal of this method eliminates the ability to mark emails as seen via this endpoint.

src/presentation/DigitalData.MessagingService.API/Controllers/EmailController.cs

@@ -113,21 +113,5 @@ public class EmailController(IMediator mediator) : ControllerBase
         else
             return Ok(emails);
     }
-
-    /// <summary>
-    /// Mark a single IMAP message as seen (read).
-    /// </summary>
-    /// <param name="command">The command containing the mailbox account identifier and the unique identifier (UID) of the message to mark as seen.</param>
-    /// <param name="cancellationToken">Cancellation token.</param>
-    /// <returns>HTTP 204 No Content.</returns>
-    [HttpPatch("seen")]
-    [ProducesResponseType(StatusCodes.Status204NoContent)]
-    [ProducesResponseType(StatusCodes.Status400BadRequest)]
-    [ProducesResponseType(StatusCodes.Status404NotFound)]
-    public async Task<IActionResult> MarkAsSeen([FromRoute] MarkEmailAsSeenCommand command, CancellationToken cancellationToken = default)
-    {
-        await mediator.Send(command, cancellationToken);
-        return NoContent();
-    }
     #endregion Receive
 }