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.ts

@@ -1,6 +1,6 @@
 import { Context } from "hono";
 import { Env } from "../types";
-import { getFeedMetadata } from "../utils/storage";
+import { FeedRepository } from "../domain/feed-repository";
 import { cacheFaviconForDomain, getCachedIcon } from "../utils/favicon-fetcher";
 
 export const FAVICON_PATH = "/favicon.svg";
@@ -40,7 +40,7 @@ export async function handleFeedFavicon(
   const feedId = c.req.param("feedId");
   if (!feedId) return projectFavicon();
 
-  const metadata = await getFeedMetadata(env.EMAIL_STORAGE, feedId);
+  const metadata = await FeedRepository.from(env).getMetadata(feedId);
   const domain = metadata?.iconDomain;
   if (!domain) return projectFavicon();