feat: [leaves] 휴가-결재 연동을 위한 DB 변경

- leaves 테이블에 approval_id 컬럼 추가 (마이그레이션)
- 휴가신청 결재 양식(approval_forms) 등록 (마이그레이션)
- Leave 모델 fillable에 approval_id 추가

database/migrations/2026_02_28_200000_add_approval_id_to_leaves_table.php

@@ -0,0 +1,24 @@
+<?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('leaves', function (Blueprint $table) {
+            $table->unsignedBigInteger('approval_id')->nullable()->after('status')->comment('연결된 결재 문서 ID');
+            $table->index('approval_id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('leaves', function (Blueprint $table) {
+            $table->dropIndex(['approval_id']);
+            $table->dropColumn('approval_id');
+        });
+    }
+};

database/migrations/2026_02_28_200100_insert_leave_approval_form.php

@@ -0,0 +1,40 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        // tenant_id=1 에 휴가신청 결재 양식 등록 (이미 존재하면 무시)
+        DB::table('approval_forms')->insertOrIgnore([
+            'tenant_id' => 1,
+            'name' => '휴가신청',
+            'code' => 'leave',
+            'category' => 'request',
+            'template' => json_encode([
+                'fields' => [
+                    ['name' => 'user_name', 'type' => 'text', 'label' => '신청자'],
+                    ['name' => 'leave_type', 'type' => 'text', 'label' => '휴가유형'],
+                    ['name' => 'period', 'type' => 'text', 'label' => '기간'],
+                    ['name' => 'days', 'type' => 'number', 'label' => '일수'],
+                    ['name' => 'reason', 'type' => 'text', 'label' => '사유'],
+                    ['name' => 'remaining_days', 'type' => 'text', 'label' => '잔여연차'],
+                ],
+            ], JSON_UNESCAPED_UNICODE),
+            'is_active' => true,
+            'created_by' => 1,
+            'created_at' => now(),
+            'updated_at' => now(),
+        ]);
+    }
+
+    public function down(): void
+    {
+        DB::table('approval_forms')
+            ->where('tenant_id', 1)
+            ->where('code', 'leave')
+            ->delete();
+    }
+};