feat: implement Learning Tracer (heartbeat duration tracking) for LPK Audit

app/Models/UserProgress.php

@@ -15,7 +15,9 @@ class UserProgress extends Model
     protected $guarded = [];
 
     protected $casts = [
+        'started_at' => 'datetime',
         'completed_at' => 'datetime',
+        'last_heartbeat_at' => 'datetime',
         'metadata' => 'array',
     ];