Fix null safety, equality contract, and silent HTTP errors in Qui application

- Prevent NullReferenceException in UpdateIndexer when indexerMapping is null
- Normalize Categories comparison in QuiIndexer.Equals for null/empty symmetry
  and add GetHashCode override to maintain equality/hash contract
- Check response status in RemoveIndexer so failed DELETEs raise HttpException

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/NzbDrone.Core/Applications/Qui/Qui.cs

@@ -145,7 +145,7 @@ public override void UpdateIndexer(IndexerDefinition indexer, bool forceSync = f
 
             var quiIndexer = BuildQuiIndexer(indexer, indexerCapabilities, indexerMapping?.RemoteIndexerId ?? 0);
 
-            var remoteIndexer = _quiProxy.GetIndexer(indexerMapping.RemoteIndexerId, Settings);
+            var remoteIndexer = _quiProxy.GetIndexer(indexerMapping?.RemoteIndexerId ?? 0, Settings);
 
             if (remoteIndexer != null)
             {

src/NzbDrone.Core/Applications/Qui/QuiIndexer.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using Newtonsoft.Json;
@@ -58,13 +59,21 @@ public bool Equals(QuiIndexer other)
                 return false;
             }
 
+            var thisCategories = (Categories ?? Enumerable.Empty<string>()).OrderBy(c => c);
+            var otherCategories = (other.Categories ?? Enumerable.Empty<string>()).OrderBy(c => c);
+
             return other.BaseUrl == BaseUrl &&
                 other.ApiKey == ApiKey &&
                 other.Name == Name &&
                 other.Enabled == Enabled &&
                 other.Priority == Priority &&
                 other.IndexerId == IndexerId &&
-                other.Categories?.OrderBy(c => c).SequenceEqual(Categories?.OrderBy(c => c) ?? Enumerable.Empty<string>()) == true;
+                otherCategories.SequenceEqual(thisCategories);
+        }
+
+        public override int GetHashCode()
+        {
+            return HashCode.Combine(BaseUrl, ApiKey, Name, Enabled, Priority, IndexerId);
         }
     }
 }

src/NzbDrone.Core/Applications/Qui/QuiProxy.cs

@@ -82,7 +82,12 @@ public QuiIndexer UpdateIndexer(QuiIndexer indexer, QuiSettings settings)
         public void RemoveIndexer(int indexerId, QuiSettings settings)
         {
             var request = BuildRequest(settings, $"{AppIndexerApiRoute}/{indexerId}", HttpMethod.Delete);
-            _httpClient.Execute(request);
+            var response = _httpClient.Execute(request);
+
+            if ((int)response.StatusCode >= 300)
+            {
+                throw new HttpException(response);
+            }
         }
 
         public ValidationFailure TestConnection(QuiSettings settings)