feat(posts): faithful (semantic) HTML rendering of post bodies

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>

backend/app/utils/html_sanitize.py

@@ -8,10 +8,23 @@ bleach is deprecated upstream and intentionally not used.
 
 import nh3
 
+# A faithful-but-safe set: enough to reproduce the SEMANTIC look of a scraped
+# post body (headings, lists, quotes, code, inline images, links, line breaks +
+# inline emphasis) without layout-injection wrappers. `div`/`span` are
+# deliberately NOT allowed — their text is preserved by nh3 and they only carry
+# source-site layout/styling we don't want to import (a test pins this).
 ALLOWED_TAGS = {
-    "p", "a", "br", "em", "strong", "b", "i", "ul", "ol", "li", "blockquote",
+    "p", "a", "br", "em", "strong", "b", "i", "u", "s",
+    "ul", "ol", "li", "blockquote",
+    "h1", "h2", "h3", "h4", "h5", "h6", "hr",
+    "pre", "code", "img", "figure", "figcaption",
 }
-ALLOWED_ATTRIBUTES = {"a": {"href", "title"}}
+ALLOWED_ATTRIBUTES = {
+    "a": {"href", "title", "target"},
+    "img": {"src", "alt", "title", "width", "height"},
+}
+# http/https for hotlinked source images today; relative URLs (e.g. a local
+# `/api/...` src once inline images are captured) pass through nh3 untouched.
 ALLOWED_URL_SCHEMES = {"http", "https"}