refactor(domain): split Icon/WebSub/Counters out of FeedRepository

FeedRepository no longer owns favicons, WebSub subscriber lists or the
monitoring counters singleton. Each concern gets its own repository
(IconRepository, WebSubSubscriptionRepository, CountersRepository),
sharing the key schema via feed-keys. KV key strings are unchanged;
counters increment policy stays in utils/stats.ts.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/domain/icon-repository.test.ts

@@ -0,0 +1,18 @@
+import { describe, it, expect } from "vitest";
+import { createMockEnv } from "../test/setup";
+import { IconRepository } from "./icon-repository";
+import type { Env } from "../types";
+
+const mockEnv = () => createMockEnv() as unknown as Env;
+
+describe("IconRepository", () => {
+  it("stores and reads favicons as text or json under the icon: key", async () => {
+    const repo = new IconRepository(mockEnv().EMAIL_STORAGE);
+    expect(await repo.getText("example.com")).toBeNull();
+    await repo.put("example.com", JSON.stringify({ data: null }), 60);
+    expect(await repo.getText("example.com")).toBe('{"data":null}');
+    expect(await repo.getJson<{ data: null }>("example.com")).toEqual({
+      data: null,
+    });
+  });
+});