feat: integrate Snyk for dependency vulnerability scanning

[MatiasOS]

Description

Integrate Snyk to continuously monitor OpenScan's dependencies for known vulnerabilities (CVEs), license issues, and supply-chain risks.

Part of the Quality Tooling Rollout (#369).

Tasks

• Set up Snyk project linked to the OpenScan repo
• Configure Snyk GitHub Actions integration for PR checks
• Add and to CI pipeline
• Configure Snyk Code for static analysis (complementary to Semgrep)
• Set up Snyk Container if/when Docker images are introduced
• Add Snyk badge to README
• Configure auto-fix PRs for non-breaking dependency bumps

Motivation

A blockchain explorer handles financial data and user assets — dependency vulnerabilities are high-impact. Snyk provides:

• Real-time CVE database with remediation advice
• License compliance scanning
• Automated fix PRs for minor/patch updates
• Broader ecosystem coverage than Dependabot alone

Notes

• Snyk has a free tier for open-source/public repos.
• Start with monitoring mode; move to blocking only for critical/high severity.