feat: 휴가 부여현황 API 추가

- leave_grants 테이블 마이그레이션 추가
- LeaveGrant 모델 생성 (annual/monthly/reward/condolence/other 유형)
- LeaveService에 getGrants, storeGrant, destroyGrant 메서드 추가
- LeaveController에 grants, storeGrant, destroyGrant 엔드포인트 추가
- GET/POST/DELETE /api/v1/leaves/grants 라우트 추가
- 연차/월차 부여 시 LeaveBalance total_days 자동 갱신

database/migrations/2025_12_24_193042_create_leave_grants_table.php

@@ -0,0 +1,35 @@
+<?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('leave_grants', function (Blueprint $table) {
+            $table->id();
+            $table->unsignedBigInteger('tenant_id')->comment('테넌트 ID');
+            $table->unsignedBigInteger('user_id')->comment('부여 대상자 ID');
+            $table->string('grant_type', 20)->comment('부여유형: annual/monthly/reward/condolence/other');
+            $table->date('grant_date')->comment('부여일');
+            $table->decimal('grant_days', 4, 1)->comment('부여일수');
+            $table->text('reason')->nullable()->comment('부여 사유');
+            $table->unsignedBigInteger('created_by')->nullable()->comment('부여자 ID');
+            $table->unsignedBigInteger('updated_by')->nullable()->comment('수정자');
+            $table->unsignedBigInteger('deleted_by')->nullable()->comment('삭제자');
+            $table->softDeletes();
+            $table->timestamps();
+
+            $table->index(['tenant_id', 'user_id'], 'idx_tenant_user');
+            $table->index('grant_date', 'idx_grant_date');
+            $table->index('grant_type', 'idx_grant_type');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('leave_grants');
+    }
+};