fix(durable_articles_three): add subject field to model

FRX template references [durable_articles_three.subject] but the
model was missing the property, causing FastReport binding to fail
and the report endpoint to return 500.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Models/budget/durable_articles_three.cs

@@ -11,6 +11,7 @@ namespace rmutr_report.Models
         public string responsible_person { get; set; }
         public string phone_no { get; set; }
         public string product { get; set; }
+        public string subject { get; set; }
         public List<durable_articles_three_detail> data  { get; set; }
         public decimal? total_amount { get; set; } 
         public string durable_type { get; set; }