feat: [approval] approvals 테이블에 drafter_read_at 컬럼 추가

- 기안자가 완료 결과를 확인했는지 추적하는 타임스탬프
- 완료함 미읽음 뱃지 기능 지원

database/migrations/2026_03_05_112000_add_drafter_read_at_to_approvals_table.php

@@ -0,0 +1,23 @@
+<?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::table('approvals', function (Blueprint $table) {
+            $table->timestamp('drafter_read_at')->nullable()->after('completed_at')
+                ->comment('기안자가 완료 결과를 확인한 시각');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('approvals', function (Blueprint $table) {
+            $table->dropColumn('drafter_read_at');
+        });
+    }
+};