feat(sales): add discount support with ledger accounting

- Add discount_type column to transactions table via migration
- Update Sale model to use fillable instead of guarded for better security
- Implement discount account ledger creation when discount is applied
- Fix net amount calculation to include discount in CreateSaleAction
- Remove unused "Exempt" column from sale transaction table
- Make discount_type required when discount is enabled in form
- Update form data mutation to properly handle discount calculations

app/Models/Sale.php

@@ -12,7 +12,14 @@ class Sale extends Model
 {
     use HasFactory;
 
-    protected $guarded = [];
+    protected $fillable = [
+        'branch_id',
+        'user_id',
+        'client_id',
+        'happened_on',
+        'reference_number',
+        'buyer'
+    ];
 
     protected $casts = [
         'happened_on' => 'date:Y-m-d',