BlobStore r2 backend (ADR-0006)

[filipeforattini]

What to build

Add an r2 BlobStore backend behind the trait. Selectable by configuration; fs remains the default. See ADR-0006.

Acceptance criteria

• r2 backend implements the BlobStore trait (put/get content-addressed).
• 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
• BlobStore s3 backend (ADR-0006) #66