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/types/index.ts

@@ -45,6 +45,7 @@ export interface FeedConfig {
 // Feed metadata interface
 export interface FeedMetadata {
   emails: EmailMetadata[];
+  iconDomain?: string; // Most recent sender's domain, used to resolve the feed icon
 }
 
 // Email metadata interface (summary info for listing)
@@ -111,6 +112,7 @@ declare global {
     put(
       key: string,
       value: string | ArrayBuffer | ReadableStream | FormData,
+      options?: { expirationTtl?: number; expiration?: number },
     ): Promise<void>;
     delete(key: string): Promise<void>;
     list(options?: {