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/websub-subscription-repository.test.ts

@@ -0,0 +1,19 @@
+import { describe, it, expect } from "vitest";
+import { createMockEnv } from "../test/setup";
+import { WebSubSubscriptionRepository } from "./websub-subscription-repository";
+import type { Env, WebSubSubscription } from "../types";
+
+const mockEnv = () => createMockEnv() as unknown as Env;
+
+describe("WebSubSubscriptionRepository", () => {
+  it("round-trips subscriptions and counts feeds with subscribers", async () => {
+    const repo = new WebSubSubscriptionRepository(mockEnv().EMAIL_STORAGE);
+    expect(await repo.get("a.b.42")).toEqual([]);
+    const subs: WebSubSubscription[] = [
+      { callbackUrl: "https://r.example/cb", expiresAt: 9999 },
+    ];
+    await repo.save("a.b.42", subs);
+    expect(await repo.get("a.b.42")).toEqual(subs);
+    expect(await repo.countKeys()).toBe(1);
+  });
+});