Add admin cooldown for events critical actions [b#089]

[greatest0fallt1me]

Description

This is a smart-contract issue for the GrantFox FWC26 campaign. Cool-off window between admin actions on events to prevent rapid abuse.

Requirements and Context

• Implement per the description above
• Add focused tests for the change
• Document any API/visible changes
• Adhere to repo's lint and code style
• Must be secure, tested, and documented

Suggested Execution

1. Fork the repo and create a branch
2. Implement changes touching:
   • contracts/events/src/admin.rs
3. Test and commit; open a PR with Closes #<issue-number> in the body

Guidelines

• Minimum 95% test coverage with cargo test
• require_auth on every state-changing entrypoint
• Overflow-safe math; no unwrap() in production paths
• Clear NatSpec-style /// rustdoc
• Timeframe: 96 hours