Set up working transactional email

[gaidheal1]

Transactional email is currently broken in production, which blocks user registration (email verification is mandatory).

Goal

Deliver a reliable, tested transactional email pipeline end-to-end — from signup confirmation to any future system emails.

Tasks

• Fix inconsistent SendGrid transactional email configuration #272 Fix inconsistent SendGrid/SMTP configuration (env vars, backend wiring, Celery worker parity, smoke-test command)
• Verify end-to-end: signup confirmation email lands in inbox in staging
• Document the chosen delivery path (SMTP vs SendGrid API client) in deployment notes

Context

See #272 for a detailed diagnosis of the current misconfiguration (mismatched credentials, malformed EMAIL_HOST, broken smoke-test command, Celery dependency).