feat: implement secure storage and encryption for third-party API tokens

[basantnema31]

Resolves #4847. This PR introduces a secure encryption utility using AES-256-GCM to securely encrypt and decrypt third-party API tokens (like GitHub PATs). It also extends the User schema to support encrypted token storage, ensuring that tokens are never stored in plaintext within the database.

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🚨 Hey @basantnema31, the CI Pipeline is failing on this PR and it has been marked as status:blocked.

Please fix the issues before this can be reviewed. Here's how:

1. Run checks locally before pushing:

npm run format:check   # Check Prettier formatting
npm run lint           # Run ESLint
npm run typecheck      # TypeScript type check
npm run test           # Run unit tests (Vitest)
npm run build          # Verify production build passes

2. Auto-fix common issues:

npm run format         # Auto-fix formatting with Prettier
npm run lint -- --fix  # Auto-fix lint errors where possible

3. Check the full failure log here:
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Once you push a fix and the CI passes, the status:blocked label will be removed automatically. 💪