refactor: invert application↔routes boundary (Track B — points 3, 6a)

- Point 3: move the feed/email storage-cleanup helpers (purgeFeedKeysStep,
  collectUnsubscribeUrls, purgeExpiredFeeds, deleteKeysWithConcurrency,
  deleteAttachmentsForEmails) out of routes/admin/helpers.ts into
  src/application/feed-cleanup.ts, so the application layer no longer imports
  from routes/. deleteFeedRecord no longer takes a Hono Context: it accepts a
  BackgroundScheduler ((task) => void) and the HTTP edge passes
  (p) => waitUntilSafe(c, p). Application/domain are now Hono-Context-free.
- Point 6a: rename the misleadingly-named Feed.rename → Feed.editDetails (it
  edits title + description), and feed-service.renameFeed → editFeedDetails.

CLAUDE.md source layout updated. 351 tests pass; tsc --noEmit clean.

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

src/routes/admin/emails.tsx

@@ -5,7 +5,7 @@ import { Layout, clampText } from "./ui";
 import {
   deleteAttachmentsForEmails,
   deleteKeysWithConcurrency,
-} from "./helpers";
+} from "../../application/feed-cleanup";
 import { FeedRepository } from "../../infrastructure/feed-repository";
 import { FeedId } from "../../domain/value-objects/feed-id";
 import {