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/Http/Requests/Document/UpdateRequest.php

@@ -27,6 +27,9 @@ public function rules(): array
             'approvers.*.user_id' => 'required_with:approvers|integer|exists:users,id',
             'approvers.*.role' => 'nullable|string|max:50',
 
+            // HTML 스냅샷
+            'rendered_html' => 'nullable|string',
+
             // 문서 데이터 (EAV)
             'data' => 'nullable|array',
             'data.*.section_id' => 'nullable|integer',