Billing→S3: config scaffolding

[Hazel-Datastax]

Scaffolding for the billing→S3 export feature. No runtime behavior yet — inert until wired in last PR.

What to do

• Add BillingS3ExportConfig (@ConfigMapping(prefix = "stargate.jsonapi.billing.s3")): enabled (default false), bucket, bucket-region, endpoint-override (Optional, for tests/LocalStack), max-events (~50), max-bytes (~2MB), max-age (flush interval), queue-capacity.
• Add software.amazon.awssdk:s3 to pom.xml.

Acceptance

• Project builds with the new dependency.