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 <[email protected]>

src/lib/email-processor.test.ts

@@ -7,7 +7,8 @@ import {
   RawAttachment,
 } from "./email-processor";
 import { getCounters } from "../utils/stats";
-import { iconKey } from "../utils/storage";
+
+const iconKey = (domain: string) => `icon:${domain}`;
 
 const VALID_FEED_ID = "apple.mountain.42";
 const VALID_TO = `${VALID_FEED_ID}@test.getmynews.app`;