[ActionMailer] Use separate SMTP infrastructure for critical/transactional/marketing

Examples:
- critical
- login
- security-related emails
- transactional
- new transactions
- most of our emails
- and _maybe_ end of month summaries. These are transactional by definition, but maybe it's better to have it run through our marketing system
- marketing
- HCB announcements

The idea is to protect our email deliverability reputation (in the case that people send junk through HCB announcements). Any bulk UGC emails should go through the marketing infra.

In addition, we should explore fallbacks. For example, if critical SMTP fails, we fallback to transactional SMTP, which fallsback to marketing??

Maybe we want to explore dedicated IP if it's reasonably priced.

Start at the ActionMailer configuration and inventory the email paths represented by critical, transactional, and marketing messages. Clarify whether bulk UGC belongs on marketing infrastructure, which fallback order is required, and whether dedicated IPs are in scope. Done requires an agreed infrastructure design; the issue names no files or tests.

Written by the indexing model from the issue text.