Documents the decision for #5: Worker-side authenticated encryption (AES-256-GCM) applied in the Worker before writing to R2, so R2 never receives plaintext or the key. Key material is held as a versioned keyring in Cloudflare Secrets Store (shared by the ingest and weekly-publish Workers). Rotation is data-loss-free via a key-id/version in each object header plus retained old key versions. Unblocks #9.
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
