Enforce S3 authentication credentials

[deepjoy]

Description

Currently, authentication is not enforced. The AWS CLI requires credentials to be set, but shoebox accepts any values.

Location: demos/pre-release/02-core-operations.sh:40

# AWS CLI configuration — auth is not enforced yet(TODO),
# but the CLI requires credentials to be set.
export AWS_ACCESS_KEY_ID=test
export AWS_SECRET_ACCESS_KEY=test

Expected behavior

Shoebox should validate S3 authentication credentials (e.g., signature verification or a simple shared-secret check) and reject unauthorized requests.