refactor(domain): introduce FeedRepository as the single KV access layer

Centralise the KV key schema and all get/put access behind a FeedRepository
class under src/domain/. Every feed/email/list/icon/websub/counter key was
previously inlined across ~12 modules with two divergent storeEmail and
addFeedToList implementations; the dead src/utils/storage.ts write path is
removed and the email key convention unified on feed:<id>:<ts>.

Behaviour-preserving: existing tests pass unchanged in logic, plus a new
feed-repository.test.ts covering CRUD, key builders, list ops and counters.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/routes/favicon.test.ts

@@ -1,9 +1,10 @@
 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";
 
+const iconKey = (domain: string) => `icon:${domain}`;
+
 function req(path: string): Request {
   return new Request(`https://test.getmynews.app${path}`);
 }