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 <noreply@anthropic.com>

src/utils/storage.ts

@@ -7,6 +7,13 @@ import {
 } from "../types";
 import { MAX_METADATA_EMAILS } from "../config/constants";
 
+/**
+ * KV key for a domain's cached favicon (shared across feeds from the same sender).
+ */
+export function iconKey(domain: string): string {
+  return `icon:${domain}`;
+}
+
 /**
  * Store email data in KV
  */