Resolve each feed's most recent sender domain and serve its favicon at
GET /favicon/:feedId, falling back to the project icon. Icons are fetched
in the background on ingestion (direct /favicon.ico then a DuckDuckGo
fallback), cached base64 in KV keyed by domain with a 1-week TTL so the
fetch only fires when absent. Exposed via RSS <image> / Atom <icon>/<logo>
and rendered in the admin feed list, plus a landing-page feature card.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>