feat: [payroll] 근로소득세 간이세액표 DB 테이블 및 시더 추가

- income_tax_brackets 테이블 마이그레이션 생성
- 2024년 국세청 간이세액표 데이터 시더 (7,117건)
- salary_from/salary_to(천원), family_count(1~11), tax_amount(원)

database/migrations/2026_02_27_200000_create_income_tax_brackets_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
+{
+    public function up(): void
+    {
+        Schema::create('income_tax_brackets', function (Blueprint $table) {
+            $table->id();
+            $table->unsignedSmallInteger('tax_year')->comment('적용 연도 (예: 2024)');
+            $table->unsignedInteger('salary_from')->comment('구간 하한 (천원 단위)');
+            $table->unsignedInteger('salary_to')->comment('구간 상한 (천원 단위)');
+            $table->unsignedTinyInteger('family_count')->comment('공제대상가족수 (1~11)');
+            $table->unsignedInteger('tax_amount')->comment('세액 (원)');
+            $table->timestamps();
+
+            $table->index(['tax_year', 'salary_from', 'salary_to', 'family_count'], 'itb_lookup_idx');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('income_tax_brackets');
+    }
+};