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>

routes/api/v1/documents.php

@@ -33,6 +33,7 @@
     Route::get('/{id}', [DocumentController::class, 'show'])->whereNumber('id')->name('v1.documents.show');
     Route::post('/', [DocumentController::class, 'store'])->name('v1.documents.store');
     Route::patch('/{id}', [DocumentController::class, 'update'])->whereNumber('id')->name('v1.documents.update');
+    Route::patch('/{id}/snapshot', [DocumentController::class, 'patchSnapshot'])->whereNumber('id')->name('v1.documents.snapshot');
     Route::delete('/{id}', [DocumentController::class, 'destroy'])->whereNumber('id')->name('v1.documents.destroy');
 
     // 결재 워크플로우