AppSync GraphQL API with DynamoDB and S3 presigned URL generation

A complete AWS CDK infrastructure-as-code project in Python 3.12 that demonstrates a serverless architecture combining:
- AppSync GraphQL API with API Key authentication for real-time data operations
- DynamoDB as the primary database for item storage and retrieval
- S3 bucket for file storage with secure presigned URL access
- Lambda resolver that generates secure, time-limited presigned URLs for S3 object access

The project includes GraphQL queries and mutations for:
- all CRUD operations for all items in DynamoDB
- Generating presigned URLs for secure S3 file downloads

The issue names no files, tests, or entry points to inspect. First determine whether this is a request to add the described Python CDK project, then define the infrastructure, GraphQL operations, and presigned-URL behavior that would constitute completion.

Written by the indexing model from the issue text.