fix(threads): mark PendingMessage thread as background, document RetentionSweep rationale

HellionChat/MessageManager.cs

@@ -66,7 +66,15 @@ internal class MessageManager : IAsyncDisposable
 
         Store = new MessageStore(DatabasePath());
 
-        PendingMessageThread = new Thread(() => ProcessPendingMessages(PendingThreadCancellationToken.Token));
+        // IsBackground = true so a stuck worker never blocks plugin unload.
+        // The worker has its own cancellation path via PendingThreadCancellationToken,
+        // and DisposeAsync waits up to 10s for cooperative shutdown. The
+        // background flag is the safety net for the case where cooperative
+        // shutdown fails to drain the queue in time.
+        PendingMessageThread = new Thread(() => ProcessPendingMessages(PendingThreadCancellationToken.Token))
+        {
+            IsBackground = true,
+        };
         PendingMessageThread.Start();
 
         ContentIdResolverHook = Plugin.GameInteropProvider.HookFromAddress<RaptureLogModule.Delegates.AddMsgSourceEntry>(RaptureLogModule.MemberFunctionPointers.AddMsgSourceEntry, ContentIdResolver);

HellionChat/Plugin.cs

@@ -691,6 +691,11 @@ public sealed class Plugin : IDalamudPlugin
             policy[(int)(ushort)type] = days;
         var defaultDays = Config.RetentionDefaultDays;
 
+        // IsBackground = true for the same reason as PendingMessageThread:
+        // a stuck sweep must never block plugin unload. RunRetentionSweepIfDue
+        // guards the run-frequency, and the sweep itself uses the framework's
+        // cooperative cancellation pattern. The background flag is the safety
+        // net if the sweep ever takes longer than expected.
         new Thread(() =>
         {
             // Bail out cheaply if a manual sweep is already in flight; the