Adds render_digest_email(kind, date_str, content_html, unsubscribe_url,
settings_url) -> tuple[str, str, str] to email_service.py, following the
same contract as render_otp_email. Includes _DIGEST_HTML_TEMPLATE with
light/dark palette from branding and _strip_html_to_text for the plain-text
fallback. Unit tests in tests/test_email_render.py cover daily, weekly, and
invalid-kind cases.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
