feat(emails): subtask 1-2 — email_templates.py foundation + 3 simple templates
- Add email_templates.py: standalone Jinja2 env, render_email_template(),
EMAIL_TEMPLATE_REGISTRY with sample_data functions for all 11 email types
- Add templates/emails/_base.html: direct transliteration of _email_wrap()
- Add templates/emails/_macros.html: email_button, heat_badge, heat_badge_sm,
section_heading, info_box macros
- Add magic_link.html, welcome.html, supplier_enquiry.html templates
- Refactor 3 handlers in worker.py to use render_email_template()
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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