feat(accounting): add Shayegan as fixed boursecu provider

Wire outbound Invoice/Document sync after confirmed ops, store sync status with timeout lock, and keep SQL credentials out of frontend responses.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/Models/Shayegan.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+class Shayegan extends Model
+{
+    protected $table = 'shayegan';
+
+    protected $guarded = [];
+
+    protected $casts = [
+        'date' => 'datetime',
+        'amount' => 'float',
+    ];
+}