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/routes/favicon.ts

@@ -1,5 +1,7 @@
 import { Context } from "hono";
 import { Env } from "../types";
+import { getFeedMetadata } from "../utils/storage";
+import { cacheFaviconForDomain, getCachedIcon } from "../utils/favicon-fetcher";
 
 export const FAVICON_PATH = "/favicon.svg";
 
@@ -13,7 +15,7 @@ export const FAVICON_SVG = `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0
   </g>
 </svg>`;
 
-export function handle(_c: Context<{ Bindings: Env }>): Response {
+function projectFavicon(): Response {
   return new Response(FAVICON_SVG, {
     headers: {
       "Content-Type": "image/svg+xml; charset=utf-8",
@@ -21,3 +23,43 @@ export function handle(_c: Context<{ Bindings: Env }>): Response {
     },
   });
 }
+
+export function handle(_c: Context<{ Bindings: Env }>): Response {
+  return projectFavicon();
+}
+
+/**
+ * Per-feed favicon. Resolves the feed's most recent sender domain and serves
+ * its cached icon; falls back to the project icon for any unresolved case
+ * (no domain, cache miss, or negative cache entry).
+ */
+export async function handleFeedFavicon(
+  c: Context<{ Bindings: Env }>,
+): Promise<Response> {
+  const env = c.env;
+  const feedId = c.req.param("feedId");
+  if (!feedId) return projectFavicon();
+
+  const metadata = await getFeedMetadata(env.EMAIL_STORAGE, feedId);
+  const domain = metadata?.iconDomain;
+  if (!domain) return projectFavicon();
+
+  const icon = await getCachedIcon(domain, env);
+  if (icon) {
+    return new Response(icon.bytes, {
+      headers: {
+        "Content-Type": icon.contentType,
+        "Cache-Control": "public, max-age=86400",
+      },
+    });
+  }
+
+  // Known domain but nothing cached yet: warm the cache in the background and
+  // serve the fallback for now.
+  try {
+    c.executionCtx.waitUntil(cacheFaviconForDomain(domain, env));
+  } catch {
+    // No ExecutionContext (e.g. tests) — fallback is served regardless.
+  }
+  return projectFavicon();
+}