Add section for reauthentication after risk events

[pankajtaneja5]

This PR adds a new section titled "Reauthentication After Risk Events" to the Session Management Cheat Sheet.

It addresses Issue #1690 by providing best practices for enforcing reauthentication following high-risk events such as:

• Password changes
• Login from new or suspicious IPs/devices
• Account recovery challenges

This aligns with modern adaptive authentication strategies to ensure session integrity and user account safety.

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[pankajtaneja5]

Closing this in favor of a cleaned-up version: #1692