fix: convert NaiveDateTime to DateTime for activity feed sorting

- date_trunc fragment returns NaiveDateTime without timezone
- Convert to UTC DateTime before returning from preseem sync query
- Fixes DateTime.compare/2 function clause error when sorting activities
- All activity timestamps now consistently use DateTime with timezone

lib/towerops/activity_feed.ex

@@ -322,7 +322,7 @@ defmodule Towerops.ActivityFeed do
       %{
         type: :sync,
         id: "sy-#{row.id}",
-        timestamp: row.timestamp,
+        timestamp: DateTime.from_naive!(row.timestamp, "Etc/UTC"),
         device_name: nil,
         site_name: nil,
         summary: "Preseem sync #{status_label}",