Add multi-provider email support (Mailgun, SES, SMTP)

[corsacca]

Summary

• Add support for Mailgun HTTP API transport (bypasses SMTP port restrictions on platforms like Railway)
• Add support for AWS SES transport
• Keep SMTP as default/fallback for backward compatibility
• Provider selection via EMAIL_PROVIDER environment variable ('smtp', 'mailgun', or 'ses')

Motivation

Railway and some other platforms block outbound SMTP connections on free/trial plans. Using Mailgun's HTTP API or AWS SES SDK bypasses these restrictions.

Configuration

Mailgun

EMAIL_PROVIDER=mailgun
MAILGUN_API_KEY=your-api-key
MAILGUN_DOMAIN=your-domain.mailgun.org
MAILGUN_HOST=api.eu.mailgun.net  # Optional, only for EU region

AWS SES

EMAIL_PROVIDER=ses
AWS_REGION=us-east-1
AWS_ACCESS_KEY_ID=your-access-key
AWS_SECRET_ACCESS_KEY=your-secret-key

SMTP (default)

EMAIL_PROVIDER=smtp  # or omit entirely
SMTP_HOST=smtp.example.com
SMTP_PORT=587
SMTP_USER=your-user
SMTP_PASS=your-password

Test plan

• Test with Mailgun HTTP API
• Test with AWS SES
• Test with SMTP (existing behavior)
• Test in development mode (MailHog)

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