Fix adversarial review findings: remove unused import and dead code

- Remove unused 'using Taskdeck.Domain.Entities' from SideEffectDtos.cs
- Remove unused hasColumnMutation variable from BuildSideEffectRows

Part of #1020: paper-review-backend-gap-side-effects

Author: Chris0Jeky

backend/src/Taskdeck.Application/DTOs/SideEffectDtos.cs

@@ -1,5 +1,3 @@
-using Taskdeck.Domain.Entities;
-
 namespace Taskdeck.Application.DTOs;
 
 public record SideEffectRowDto(

backend/src/Taskdeck.Application/Services/SideEffectAnalyzer.cs

@@ -61,9 +61,6 @@ internal static IReadOnlyList<SideEffectRow> BuildSideEffectRows(
         bool hasActiveWebhooks)
     {
         bool hasCardMutation = operations.Any(op => CardMutatingActions.Contains(op.ActionType));
-        bool hasColumnMutation = operations.Any(op =>
-            string.Equals(op.TargetType, "column", StringComparison.OrdinalIgnoreCase) &&
-            string.Equals(op.ActionType, "create_column", StringComparison.OrdinalIgnoreCase));
         bool hasAnyOperation = operations.Count > 0;
 
         return new List<SideEffectRow>