Add `cargo audit` + `cargo deny` to `contracts/` CI pipeline

[snowrugar-beep]

• Problem Statement. ci.yml builds, lints, and tests but does
  not flag CVEs in crates.
• Why it matters. Faster CVE detection, less churn.
• Expected Outcome. Add a contracts-audit job running
  cargo audit and cargo deny check.
• Acceptance Criteria.
  1. CI fails on HIGH/CRITICAL advisories.
  2. deny.toml committed with allow-list.
• Files / Modules. .github/workflows/ci.yml,
  contracts/deny.toml.
• Dependencies. cargo-audit, cargo-deny.
• Difficulty. Easy.
• Effort. 0.5 day.
• Labels. security, ci, contracts, good first issue.

Frontend UX & Accessibility

[GBOYEE]

Hey! The contracts/ CI builds and tests but doesn't flag CVEs in crates. I'll add a contracts-audit job to .github/workflows/ci.yml running cargo audit and cargo deny check, plus commit a contracts/deny.toml allow-list. CI will then fail on HIGH/CRITICAL. Mind if I take this on? Should be quick, about an hour.

[Moonwalker-rgb]

Hi maintainer,

I would like to request assignment for the smart contract security ticket to integrate automated vulnerability and dependency scanning into our CI pipeline.

I will add a contracts-audit job to .github/workflows/ci.yml running cargo audit and cargo deny check, and commit a configured contracts/deny.toml allow-list to ensure builds fail on high or critical security advisories.

[grantfox-oss]

🦊 GrantFox — @Moonwalker-rgb has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #136)
2. Your PR will be reviewed by the VertexChainLabs maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Moonwalker-rgb's PR #371 was approved and merged by @snowrugar-beep.

🏆 @Moonwalker-rgb: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (697 total points). Track your full progress on GrantFox.

👏 Great work, @Moonwalker-rgb! Keep contributing to VertexChainLabs.