feat(email-templates): tests, docs, and fix quote_verification sample data (subtask 8)

- Add 50 tests in test_email_templates.py:
  - TestRenderEmailTemplate: all 11 registry templates render in EN + DE
    without error; checks DOCTYPE, wordmark, font, CTA color, template-
    specific content (heat badges, brief rows, weekly digest loop, etc.)
    and registry structure
  - TestEmailGalleryRoutes: access control, gallery list (all labels
    present, preview links), preview pages (EN/DE/nonexistent/invalid-lang),
    compose preview endpoint (plain + wrapped + empty body)
- Fix _quote_verification_sample: add missing recap_parts key — StrictUndefined
  raised on the {% if recap_parts %} check when the variable was absent
- Update CHANGELOG.md: document email template system (renderer, base,
  macros, 11 templates, registry, gallery, compose preview, removed helpers)
- Update PROJECT.md: add email template system + gallery to Done section

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

PROJECT.md

@@ -107,6 +107,8 @@
 - [x] Task queue management (list, retry, delete)
 - [x] Lead funnel stats on admin dashboard
 - [x] Email hub (`/admin/emails`) — sent log, inbox, compose, audiences, delivery event tracking via Resend webhooks
+- [x] **Email template system** — 11 transactional emails as Jinja2 templates (`emails/*.html`); standalone `render_email_template()` renderer works in worker + admin; `_base.html` + `_macros.html` shared shell; `EMAIL_TEMPLATE_REGISTRY` with sample data for gallery previews; `_email_wrap()` / `_email_button()` helpers removed
+- [x] **Admin email gallery** (`/admin/emails/gallery`) — card grid of all templates, EN/DE preview in sandboxed iframe, "View in sent log" cross-link; compose page now has HTMX live preview pane
 - [x] **pSEO Engine tab** (`/admin/pseo`) — content gap detection, data freshness signals, article health checks (hreflang orphans, missing build files, broken scenario refs), generation job monitoring with live progress bars
 - [x] **Marketplace admin dashboard** (`/admin/marketplace`) — lead funnel, credit economy, supplier engagement, live activity stream, inline feature flag toggles
 - [x] **Lead matching notifications** — `notify_matching_suppliers` task on quote verification + `send_weekly_lead_digest` every Monday; one-click CTA token in forward emails