fix: chat.php — add 'after' parameter for incremental message polling

The client sends `after: lastTime` to fetch only new messages since the
last poll. Server now handles this with `created_at gt.{after}` filter.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api/chat.php

@@ -25,6 +25,7 @@ if ($method === 'GET') {
         if (!$friendship) jsonError('Not friends');
 
         $before = $_GET['before'] ?? null;
+        $after = $_GET['after'] ?? null;
         $limit = min(intval($_GET['limit'] ?? 50), 100);
 
         $params = [
@@ -33,7 +34,10 @@ if ($method === 'GET') {
             'order' => 'created_at.desc',
             'limit' => $limit
         ];
-        if ($before) {
+        if ($after) {
+            $params['created_at'] = 'gt.' . $after;
+            $params['order'] = 'created_at.asc';
+        } elseif ($before) {
             $params['created_at'] = 'lt.' . $before;
         }