AppSync GraphQL API with DynamoDB and RDS Aurora PostgreSQL data sources, using VTL mapping templates.

Ported from the original localstack-pro-samples repo.

1. Creates an AppSync GraphQL API with API key authentication
2. Creates a DynamoDB table and an RDS Aurora PostgreSQL cluster as data sources
3. Wires up VTL resolvers for mutations (insert) and queries (scan/select) against both backends
4. GraphQL schema exposes addPostDDB, getPostsDDB, addPostRDS, getPostsRDS

• AppSync GraphQL API + API key
• GraphQL schema with Query, Mutation, and Subscription types
• DynamoDB table (posts)
• RDS Aurora PostgreSQL cluster
• Secrets Manager secret (RDS password)
• IAM role for AppSync
• 2 data sources (DynamoDB, RDS)
• 4 resolvers with VTL mapping templates

DynamoDB Scan VTL resolver returns empty data. The getPostsDDB query uses a standard DynamoDB Scan VTL template ($util.toJson($context.result)) identical to the original sample. LocalStack returns {"data": {}} instead of the scan results. The mutation (addPostDDB) works correctly, and the equivalent RDS operations (addPostRDS, getPostsRDS) both work. This is specific to LocalStack's AppSync VTL engine handling of DynamoDB Scan operations.

This causes test_graphql_ddb_query to fail across all 4 IaC methods (4 failures out of 52 total tests).