feat(favicon): per-feed icon from the last sender's domain

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 <[email protected]>

src/utils/feed-generator.ts

@@ -23,9 +23,14 @@ function buildFeed(
   feedId: string,
   selfUrl?: { rss?: string; atom?: string },
 ): Feed {
+  const iconUrl = `${baseUrl}/favicon/${feedId}`;
   const feed = new Feed({
     title: feedConfig.title,
     description: feedConfig.description || "",
+    // Per-feed icon derived from the last sender's domain (self-falls-back to
+    // the project icon). image → RSS <image>/Atom <logo>; favicon → Atom <icon>.
+    image: iconUrl,
+    favicon: iconUrl,
     // Computed dynamically so the id is always canonical regardless of what
     // was stored in KV at feed-creation time (which may have used a stale domain).
     id: `${baseUrl}/rss/${feedId}`,