feat: [문서] rendered_html 스냅샷 저장 + snapshot 엔드포인트

- Document upsert에 rendered_html 필드 추가
- Lazy Snapshot API (snapshot_document_id resolve)
- UpsertRequest rendered_html 검증 추가
- DocumentTemplateSection description 컬럼

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

database/migrations/2026_03_06_212500_add_description_to_document_template_sections.php

@@ -0,0 +1,22 @@
+<?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('document_template_sections', function (Blueprint $table) {
+            $table->text('description')->nullable()->after('title')->comment('섹션 설명/안내문');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('document_template_sections', function (Blueprint $table) {
+            $table->dropColumn('description');
+        });
+    }
+};