Add email rendering/delivery for dashboard Excel exports: success and failure
HTML bodies (inline styles only, no logo, matching report-notification emails),
subject lines prefixed with EMAIL_REPORTS_SUBJECT_PREFIX, and a send wrapper
over send_email_smtp. Dashboard titles and chart names are HTML-escaped to
prevent injection. Adds unit tests.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
