Docs: document sign-out revocation and the 30-day session cap

[SandObserver]

Summary

• Security: add that a session ends 30 days after sign-in even while in use, or after the idle lifetime if longer.
• Security: add that signing out ends the session on the server, so a copy of its cookie stops working.
• Docker installation: extend the SESSION_MAX_AGE_DAYS row with the 30-day cap.

Testing
npm run build and npm run check pass. Text only; checked against api/src/auth.js and docker-compose.yml in the app repo.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

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