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

.devdbrc

@@ -0,0 +1,11 @@
+[
+    {
+		"name": "My test MySQL database",
+		"type": "mysql",
+		"host": "192.168.100.105",
+		"port": "3306",
+		"username": "root",
+		"password": "root",
+		"database": "mkm_admin" 
+	}
+]