Portfolio: report and suspend pages #47

Part of #40. Depends on #43 and #44.

Scope

• A 'Report this page' link on every portfolio. It opens an email to the site contact with the page address filled in, like the current contact form.
• An operator procedure to suspend and restore a portfolio, either a documented command or a protected operator endpoint. A suspended page returns 404, and the student cannot republish it until it is restored.
• Document the procedure in backend/README.md.

Done when

Backend tests cover suspension, and the public page returns 404 for a suspended portfolio.