fix: hide successful Preseem syncs from activity feed

Only show failed Preseem syncs in the activity feed to reduce noise.
Successful syncs happen every 10-20 minutes and clutter the dashboard
without providing useful information to users.

Users can still see all sync history (including successful syncs) in
the Preseem integration settings page if needed.

lib/towerops/activity_feed.ex

@@ -299,6 +299,8 @@ defmodule Towerops.ActivityFeed do
     query =
       from(sl in SyncLog,
         where: sl.organization_id == ^org_id,
+        # Only show failed syncs in activity feed, not successful ones
+        where: sl.status != "success",
         select: %{
           id: sl.id,
           timestamp: sl.inserted_at,