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.test.ts

@@ -1,12 +1,19 @@
 import { describe, it, expect } from "vitest";
 import worker from "../index";
 import { createMockEnv } from "../test/setup";
+import { iconKey } from "../utils/storage";
 import type { Env } from "../types";
 
 function req(path: string): Request {
   return new Request(`https://test.getmynews.app${path}`);
 }
 
+const PNG = new Uint8Array([0x89, 0x50, 0x4e, 0x47, 9, 8, 7]);
+
+function toBase64(bytes: Uint8Array): string {
+  return btoa(String.fromCharCode(...bytes));
+}
+
 describe("project favicon", () => {
   it("serves an SVG favicon at /favicon.svg", async () => {
     const env = createMockEnv() as unknown as Env;
@@ -27,3 +34,58 @@ describe("project favicon", () => {
     expect(body).toContain("<svg");
   });
 });
+
+describe("per-feed favicon", () => {
+  it("serves the cached domain icon when available", async () => {
+    const env = createMockEnv() as unknown as Env;
+    await env.EMAIL_STORAGE.put(
+      "feed:abc:metadata",
+      JSON.stringify({ emails: [], iconDomain: "github.com" }),
+    );
+    await env.EMAIL_STORAGE.put(
+      iconKey("github.com"),
+      JSON.stringify({ data: toBase64(PNG), contentType: "image/png" }),
+    );
+
+    const res = await worker.fetch(req("/favicon/abc"), env);
+    expect(res.status).toBe(200);
+    expect(res.headers.get("Content-Type")).toBe("image/png");
+    expect(new Uint8Array(await res.arrayBuffer())).toEqual(PNG);
+  });
+
+  it("falls back to the project SVG when the feed has no icon domain", async () => {
+    const env = createMockEnv() as unknown as Env;
+    await env.EMAIL_STORAGE.put(
+      "feed:abc:metadata",
+      JSON.stringify({ emails: [] }),
+    );
+
+    const res = await worker.fetch(req("/favicon/abc"), env);
+    expect(res.status).toBe(200);
+    expect(res.headers.get("Content-Type")).toMatch(/^image\/svg\+xml/);
+    expect(await res.text()).toContain("<svg");
+  });
+
+  it("falls back to the project SVG for a negative cache entry", async () => {
+    const env = createMockEnv() as unknown as Env;
+    await env.EMAIL_STORAGE.put(
+      "feed:abc:metadata",
+      JSON.stringify({ emails: [], iconDomain: "nope.test" }),
+    );
+    await env.EMAIL_STORAGE.put(
+      iconKey("nope.test"),
+      JSON.stringify({ data: null, contentType: "" }),
+    );
+
+    const res = await worker.fetch(req("/favicon/abc"), env);
+    expect(res.status).toBe(200);
+    expect(res.headers.get("Content-Type")).toMatch(/^image\/svg\+xml/);
+  });
+
+  it("falls back to the project SVG for an unknown feed", async () => {
+    const env = createMockEnv() as unknown as Env;
+    const res = await worker.fetch(req("/favicon/missing"), env);
+    expect(res.status).toBe(200);
+    expect(res.headers.get("Content-Type")).toMatch(/^image\/svg\+xml/);
+  });
+});