feat(mutual-aid): add request timestamps and expiry policy

[Uyoxy]

Closes #408

Summary

Adds lifecycle timestamps and expiry enforcement for Mutual Aid help requests.

This introduces request creation timestamps and optional expiry timestamps, enabling deterministic handling of stale requests. Contract logic now validates request expiry during donation and cancellation flows, preventing interactions with expired requests.

What's Changed

• Extended HelpRequest with created_at
• Added optional expires_at field
• Set timestamps when posting help requests
• Enforced expiry validation during donations
• Updated cancellation logic to respect expiry status
• Defined behaviour for expired but unfunded requests
• Added unit tests covering:
  • newly created requests
  • expired requests
  • funded requests
  • donation behaviour before and after expiry

Testing

cargo fmt --all
cargo clippy --all-targets --all-features -- -D warnings
cargo test --workspace
pnpm lint
pnpm test