Dashboard security headers → moved

This runbook moved into the public docs site so customers and security reviewers can read it without cloning the repo.

Canonical location: docs/src/content/docs/security/dashboard-headers.md

Published at: https://docs.celestialintelligence.co/security/dashboard-headers/ (rebuild on the next pnpm docs:build + Pages deploy).

The middleware itself lives at the top of packages/dashboard-server/src/server.ts. Update both this pointer and the canonical doc when the policy changes.