BlobStore s3 backend (ADR-0006)

[filipeforattini]

What to build

Add an s3 BlobStore backend behind the trait from the expand step. Selectable by configuration; absent config keeps the fs default. See ADR-0006.

Acceptance criteria

• s3 backend implements the BlobStore trait (put/get content-addressed).
• Backend selectable by configuration; fs remains the default.
• Tests cover the backend against a mock/local endpoint.

Blocked by

• BlobStore trait + content-addressed fs backend (ADR-0006) #63