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');
+    }
+};

database/seeders/IncomeTaxBracketSeeder.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace Database\Seeders;
+
+use Illuminate\Database\Seeder;
+use Illuminate\Support\Facades\DB;
+
+class IncomeTaxBracketSeeder extends Seeder
+{
+    public function run(): void
+    {
+        $taxYear = 2024;
+
+        // 기존 해당 연도 데이터 삭제 후 재생성
+        DB::table('income_tax_brackets')->where('tax_year', $taxYear)->delete();
+
+        // JSON 데이터 로드 (format: [[salary_from, salary_to, [tax1..tax11]], ...])
+        $jsonPath = database_path('seeders/data/income_tax_2024.json');
+        $data = json_decode(file_get_contents($jsonPath), true);
+
+        $records = [];
+        $now = now();
+
+        foreach ($data as $row) {
+            [$salaryFrom, $salaryTo, $taxes] = $row;
+
+            for ($fc = 1; $fc <= 11; $fc++) {
+                $records[] = [
+                    'tax_year' => $taxYear,
+                    'salary_from' => $salaryFrom,
+                    'salary_to' => $salaryTo,
+                    'family_count' => $fc,
+                    'tax_amount' => $taxes[$fc - 1],
+                    'created_at' => $now,
+                    'updated_at' => $now,
+                ];
+            }
+        }
+
+        // 500건씩 청크 삽입
+        foreach (array_chunk($records, 500) as $chunk) {
+            DB::table('income_tax_brackets')->insert($chunk);
+        }
+
+        $this->command->info("Income tax brackets seeded: {$taxYear}년 ".count($records).'건');
+    }
+}