feat: 매출 세금계산서/거래명세서 토글 API 연동

- UpdateSaleRequest: tax_invoice_issued, transaction_statement_issued 필드 추가
- SaleService: 토글 필드 업데이트 로직 추가 (canEdit 우회)
- Sale 모델: fillable에 토글 필드 추가
- 마이그레이션: sales 테이블에 토글 컬럼 추가

database/migrations/2025_12_24_151437_add_invoice_flags_to_sales_table.php

@@ -0,0 +1,29 @@
+<?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('sales', function (Blueprint $table) {
+            $table->boolean('tax_invoice_issued')->default(false)->after('status')->comment('세금계산서 발행완료');
+            $table->boolean('transaction_statement_issued')->default(false)->after('tax_invoice_issued')->comment('거래명세서 발행완료');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('sales', function (Blueprint $table) {
+            $table->dropColumn(['tax_invoice_issued', 'transaction_statement_issued']);
+        });
+    }
+};