feat: auto-email sending via SMTP (--send flag)

Closes #45

- New redmine_reporter/mailer.py: SMTP email sending with
  {author}/{period} template substitution, MIME attachment
  with correct content-type per file extension
- Config.get_email_config(): returns EmailConfig from YAML
  or None when not configured
- CLI --send flag: sends report after generation, works with
  --output, --commit, or standalone (saves to template path)
- 31 new tests (22 mailer + 6 CLI + 3 config)
- 249/249 tests passing, ruff clean, mypy clean

.gitignore

@@ -85,6 +85,7 @@ secrets.json
 # Temporary files
 *.tmp
 *.bak
+docs/superpowers
 
 # Just in case
 .~*