[BE-92] Consolidate the duplicate mail/ and email/ modules

[yusuftomilola]

Overview

The backend has two overlapping email concerns: backend/src/mail/ (1 file) and backend/src/email/ (2 files, and EmailModule is the one registered in app.module.ts). Dependencies include @sendgrid/mail, nodemailer, nodemailer-mjml, mjml, and handlebars — more than one templating and transport path.

Tasks

• Pick one module (email/, already registered) and one transport strategy; document why.
• Move any unique logic out of mail/ and delete the directory.
• Settle on a single template engine (MJML+Handlebars or plain Handlebars) and convert stragglers.
• Provide one EmailService interface: send({ to, template, context }), with a dev transport that writes to disk/console instead of sending.
• Remove now-unused mail dependencies from package.json.

Acceptance Criteria

• Only one email module remains and every caller uses it.
• Local development sends no real email by default.
• Unused mail packages are removed from package.json.

Notes for Contributors

Comment below to be assigned.