Add linked suggestions with synced voting

Endpoints/EndpointHelpers.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using GameList.Data;
 using GameList.Domain;
 using Microsoft.AspNetCore.Mvc;
@@ -163,4 +164,35 @@ internal static class EndpointHelpers
         ResultsOpen = false,
         UpdatedAt = DateTimeOffset.UnixEpoch
     };
+
+    public static Dictionary<int, int> BuildLinkRoots(IEnumerable<(int Id, int? ParentId)> items)
+    {
+        var parentMap = items.ToDictionary(x => x.Id, x => x.ParentId);
+        var roots = new Dictionary<int, int>();
+        foreach (var id in parentMap.Keys)
+        {
+            roots[id] = FindRootId(id, parentMap);
+        }
+        return roots;
+    }
+
+    public static int FindRootId(int suggestionId, IReadOnlyDictionary<int, int?> parentMap)
+    {
+        var current = suggestionId;
+        var visited = new HashSet<int>();
+
+        while (parentMap.TryGetValue(current, out var parent) && parent is int p && !visited.Contains(p))
+        {
+            visited.Add(current);
+            current = p;
+        }
+
+        return current;
+    }
+
+    public static List<int> LinkedIdsFor(int suggestionId, IReadOnlyDictionary<int, int> rootIndex)
+    {
+        if (!rootIndex.TryGetValue(suggestionId, out var root)) return new();
+        return rootIndex.Where(kv => kv.Value == root).Select(kv => kv.Key).ToList();
+    }
 }