feat: add CI security scanning

[Mosas2000]

Summary

Adds a dedicated CI security-scanning job to catch secrets, dependency vulnerabilities, and common SAST findings across the monorepo.

Changes

• Run gitleaks with full git history.
• Audit Rust dependencies with cargo-audit.
• Check Go modules with govulncheck and gosec.
• Audit JavaScript dependencies with pnpm audit.
• Audit Python dependencies with pip-audit and run bandit.

Closes #245

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