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/test/setup.ts

@@ -29,7 +29,12 @@ class MockKV {
     return type === "json" ? JSON.parse(value) : value;
   }
 
-  async put(key: string, value: any) {
+  async put(
+    key: string,
+    value: any,
+    _options?: { expirationTtl?: number; expiration?: number },
+  ) {
+    // TTL options are accepted for API parity but not simulated in tests.
     this.store.set(
       key,
       typeof value === "string" ? value : JSON.stringify(value),