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/Controllers/Api/V1/Documents/DocumentController.php

@@ -74,6 +74,22 @@ public function destroy(int $id): JsonResponse
         }, __('message.deleted'));
     }
 
+    /**
+     * rendered_html 스냅샷 저장 (Lazy Snapshot)
+     * PATCH /v1/documents/{id}/snapshot
+     */
+    public function patchSnapshot(int $id, UpdateRequest $request): JsonResponse
+    {
+        return ApiResponse::handle(function () use ($id, $request) {
+            $renderedHtml = $request->validated()['rendered_html'] ?? null;
+            if (! $renderedHtml) {
+                throw new \Symfony\Component\HttpKernel\Exception\BadRequestHttpException('rendered_html is required');
+            }
+
+            return $this->service->patchSnapshot($id, $renderedHtml);
+        }, __('message.updated'));
+    }
+
     // =========================================================================
     // FQC 일괄생성 (제품검사)
     // =========================================================================