feat: integrate Mailjet for transactional email (SMTP)

[GRACENOBLE]

Summary

Add Mailjet as the transactional email provider for sending system emails (welcome, password reset, notifications) from the Go backend.

Scope

Backend (Go)

• Add the Mailjet Go SDK (github.com/mailjet/mailjet-apiv3-go/v4) or send via Mailjet SMTP using Go's net/smtp
• Add an EmailSender interface in usecase/ and implement it in infrastructure/email/mailjet/
• Wire the client in server.go using MAILJET_API_KEY and MAILJET_SECRET_KEY env vars
• Implement at least one concrete email (e.g. SendWelcomeEmail) as a reference — natural trigger point from QStash jobs (feat: integrate Asynq + Redis Streams for background jobs and event-driven messaging #18) or Firebase Auth sign-up (feat: add Firebase Authentication across all app layers #15)
• Set a FROM_EMAIL and FROM_NAME env var for the sender identity

Email templates

• Store templates as Go text/template or html/template files under internal/infrastructure/email/templates/
• Render templates server-side before sending — no external template hosting required at this stage

Acceptance criteria

• Backend can send an email via Mailjet API (not raw SMTP) with a rendered HTML body
• SendWelcomeEmail function works end-to-end in a local integration test (or against Mailjet sandbox)
• MAILJET_API_KEY, MAILJET_SECRET_KEY, FROM_EMAIL, and FROM_NAME added to backend/.env.example with placeholders and comments
• EmailSender interface is unit-tested with a mock; integration test hits the real Mailjet sandbox API
• Docs added to backend/docs/email.md covering the interface, template pattern, and env var setup

Dependencies

• feat: add Firebase Authentication across all app layers #15 Firebase Auth — welcome email triggered on sign-up
• feat: integrate Asynq + Redis Streams for background jobs and event-driven messaging #18 QStash — email delivery can be offloaded as a background job