P0-1 partial + AppImage fallback: clean placeholder for every broken image

P0-1 (featured-image fallback path):
- AppImage.tsx: default fallbackSrc changed from /assets/images/no_image.png
  to /assets/images/article-placeholder.svg, so any article image that
  loads but 404s gets the new clean dark gradient placeholder instead of
  the old grey BB_Gray-style box. The smoke-test article
  (matrox-video-and-amagi-collaborate-...) had a featured_image URL that
  404s on broadcastbeat.com; it now renders the new placeholder.

P0-1 batch (Wayback recovery) is NOT done — see PENDING_ads.md. The
attempted backfill via Wayback found no snapshots for the Matrox smoke
article (it is a 2026 NAB story Wayback never crawled), and the batch
loop tripped a 403 from one of its outbound calls. Proper backfill
needs per-vendor press-kit scraping and a new bb-media storage bucket.

PENDING_ads.md updated with: featured-image backfill TODOs, the banners
that still need local creatives, the hardcoded slug refactor for
NewsTicker / FeaturedBento / ArticleFeed, and the SEO re-enable gate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/ui/AppImage.tsx

@@ -35,7 +35,7 @@ const AppImage = memo(function AppImage({
     fill = false,
     sizes,
     onClick,
-    fallbackSrc = '/assets/images/no_image.png',
+    fallbackSrc = '/assets/images/article-placeholder.svg',
     loading = 'lazy',
     unoptimized = false,
     ...props