feat: implement email notification for internship status updates and add missing student relationship

backend/app/Models/Internship.php

@@ -40,6 +40,11 @@ class Internship extends Model
         ];
     }
 
+    public function student()
+    {
+        return $this->belongsTo(User::class, 'user_id');
+    }
+
     public function company()
     {
         return $this->belongsTo(Company::class, 'company_id');