chore: remove unused joi and sequelize dependencies

[PierreBrisorgueil]

Problem

Two dependencies are no longer used but still in package.json:

joi (^3.18.1)

• Stack migrated to Zod for all schema validation
• lib/helpers/joi.js is a stub file with no imports anywhere
• Zod helper (lib/helpers/zod.js) is the active replacement

sequelize (^6.37.8)

• lib/services/sequelize.js is fully commented out
• Stack uses Mongoose exclusively
• No SQL database configured

Proposed change

1. npm uninstall joi sequelize
2. Delete lib/helpers/joi.js
3. Delete lib/services/sequelize.js
4. Verify no remaining imports

Why

Dead dependencies increase install time, attack surface, and audit noise.