ci: integrate Trivy Docker image vulnerability scanning

[AnzalKhan16]

📌 Overview
Integrated Trivy vulnerability scanning into the GitHub Actions CI workflow to automatically scan our Docker images (Frontend, Backend, ML Service) before they are deployed. The workflow is configured to block CI if any HIGH or CRITICAL vulnerabilities are found, and SARIF reports are uploaded directly to the GitHub Security Code Scanning tab. I also added a section in SECURITY.md detailing the automated Docker vulnerability scanning.

🛠️ Type of Change
[ ] ⛓️ Smart Contract (Solidity changes, Gas optimization)
[ ] 💻 Frontend (UI/UX, React components, Tailwind)
[ ] ⚙️ Backend (API routes, MongoDB schemas, Middleware)
[x] 📄 Documentation (README, Roadmap updates, SECURITY.md)
[x] 🧪 Testing (Hardhat tests, Jest/Vitest, CI/CD Pipeline & Security Scanning)

🔗 Related Issue
Closes #836

🧪 Testing & Verification
Smart Contracts: npx hardhat test passed? (NA)
Frontend: Verified on Mobile/Desktop responsiveness? (NA)
Integration: Verified ethers.js connectivity with local/testnet node? (NA)

📸 Screenshots / Demos
(N/A - CI configuration updates only)

✅ PR Checklist
[x] My code follows the project's style guidelines.
[x] I have commented my code, particularly in complex areas (e.g., Smart Contract logic).
[x] I have updated the documentation accordingly.
[x] My changes generate no new warnings.

[vercel]

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