Move table creation logic into dynamodbstore.EnsureTables so downstream
projects (protosource-auth, todoapp) can import it instead of
duplicating. Tables are created with deletion protection, PITR, and TTL
enabled (PITR/TTL best-effort for DynamoDB Local compatibility).

Refactor testdynamo-setup CLI to delegate create to the library function
and use the exported NumGSIs constant.