perf(dbviewer): dispatch FTS filter to worker thread

FullTextSearch + LoadByGuids could stall the draw thread for 100-300 ms
on large databases with a popular search term. The two hot trigger sites
(FTS toggle, search input) now route via TriggerFilterRefresh, which
dispatches the FTS path to Task.Run; the in-memory page-filter path
stays inline because it is sub-ms on the loaded page array.

_ftsFilterSeq is bumped per trigger so a late worker recognises itself
as stale and drops its result instead of overwriting a newer one. The
date/channel and history workers already lived on Task.Run and are
untouched.

Surfaced during the v1.4.8 pre-tag review.

HellionChat/Ui/DbViewer.cs

@@ -2,6 +2,7 @@
 using System.Globalization;
 using System.Numerics;
 using System.Text;
+using System.Threading;
 using Dalamud.Bindings.ImGui;
 using Dalamud.Game.Text.SeStringHandling.Payloads;
 using Dalamud.Interface;
@@ -44,6 +45,10 @@ public class DbViewer : Window
 
     private bool IsProcessing;
     private long ProcessingStart = Environment.TickCount64;
+
+    // Bumped per trigger so a late worker drops itself instead of overwriting
+    // a newer result.
+    private long _ftsFilterSeq;
     private (DateTime Min, DateTime Max, int Page, bool Local, int ChannelCount) LastProcessed;
 
     private string MinDateString = "";
@@ -249,7 +254,7 @@ public class DbViewer : Window
         using (ImRaii.Disabled(!ftsReady))
         {
             if (ImGui.Checkbox(HellionStrings.DbViewer_FullTextToggle, ref UseFullTextSearch))
-                Filtered = Filter(Messages);
+                TriggerFilterRefresh();
         }
         ImGuiUtil.HelpMarker(
             ftsReady
@@ -267,7 +272,7 @@ public class DbViewer : Window
                 30
             )
         )
-            Filtered = Filter(Messages);
+            TriggerFilterRefresh();
 
         // Third row
 
@@ -471,6 +476,34 @@ public class DbViewer : Window
         }
     }
 
+    // FTS path hits SQLite per keystroke -- dispatch to a worker, drop stale
+    // results via _ftsFilterSeq. Page-filter path is in-memory LINQ, stays
+    // inline.
+    private void TriggerFilterRefresh()
+    {
+        if (!UseFullTextSearch || !Plugin.MessageManager.Store.IsFtsIndexBuilt)
+        {
+            Filtered = Filter(Messages);
+            return;
+        }
+
+        var snapshot = Messages;
+        var mySeq = Interlocked.Increment(ref _ftsFilterSeq);
+        Task.Run(() =>
+        {
+            try
+            {
+                var result = Filter(snapshot);
+                if (Interlocked.Read(ref _ftsFilterSeq) == mySeq)
+                    Filtered = result;
+            }
+            catch (Exception ex)
+            {
+                Plugin.LogProxy.Error(ex, "FTS filter worker failed");
+            }
+        });
+    }
+
     private ConcurrentStack<Message> Filter(Message[] messages)
     {
         if (SimpleSearchTerm == "")