docs: add tempChatRetention config documentation

[ConstantTime]

Summary

Adds documentation for the new temporaryChatRetention configuration option introduced in LibreChat PR #7917.

This feature allows administrators to configure how long temporary chats are retained before automatic deletion, replacing the hardcoded 30-day limit with flexible configuration from 1 hour to 1 year.

Changes Made

• Added temporaryChatRetention to interface configuration subkeys table
• Created dedicated documentation section explaining:
  • Configuration methods (environment variable vs config file)
  • Validation rules (min: 1 hour, max: 8760 hours, default: 720 hours)
  • Common retention periods with examples
  • Priority system and automatic fallback behavior

Example Configuration

interface:
  # Retain temporary chats for 7 days (168 hours)
  temporaryChatRetention: 168

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