The immutable Cache-Control added for the reader preload work was
`public` unconditionally. Under PRIVATE_MODE, /files is auth-gated, so a
shared cache/CDN would store the blob and serve it to unauthenticated
clients, defeating the gate. Emit `private, ...` when private_mode is on,
`public, ...` otherwise.
Found in the security audit; regression from 0.124.3. Bump to 0.124.9.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>