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>

app/Models/Documents/Document.php

@@ -73,6 +73,7 @@ class Document extends Model
         'linkable_id',
         'submitted_at',
         'completed_at',
+        'rendered_html',
         'created_by',
         'updated_by',
         'deleted_by',

app/Models/Documents/DocumentTemplateSection.php

@@ -22,6 +22,7 @@ class DocumentTemplateSection extends Model
     protected $fillable = [
         'template_id',
         'title',
+        'description',
         'image_path',
         'sort_order',
     ];