Phase 1 of milestone #64. The body is captured (Phase 0) but was shown as
plain text. Now:
- html_sanitize.py: widen the allowlist to a faithful-but-safe set — headings,
inline images, lists, blockquote, hr, code/pre, figure, links (div/span stay
stripped; their text is preserved). Benefits the existing ProvenancePanel too.
- post_feed_service.get_post: add sanitized `description_html` to the DETAIL
response (the feed list stays lightweight plain text by design).
- PostCard.vue: render description_html via v-html once expanded (fetched with
detail); collapsed + no-detail fallback stay plain text. Styled close to the
source (headings, images max-width, accent links, lists, quotes, code).
Tests: sanitizer (headings/img/lists survive, img javascript: src dropped);
get_post returns sanitized description_html.
Refs FC #830.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
bvandeusen