Add development SMTP server (MailHog) for email testing

[nanaf6203-bit]

Problem

No development email configuration. Developers may accidentally send test emails to real addresses or have email features untestable.

Expected Outcome

MailHog or similar SMTP server in docker-compose for development email testing.

Acceptance Criteria

• MailHog service in docker-compose.yml
• Dev env vars point to MailHog
• Web UI for viewing sent emails
• Documentation for email testing

Files Affected

docker-compose.yml, .env.example, README.md

Difficulty

Easy

Estimated Effort

Small (1 hour)

Labels

developer-experience, email, enhancement