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

database/migrations/2026_02_17_175416_add_discount_type_on_transaction_table.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('transactions', function (Blueprint $table) {
+            $table->string('discount_type')->after('with_discount')->nullable();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('transactions', function (Blueprint $table) {
+            $table->dropColumn('discount_type');
+        });
+    }
+};