refactor: split src into domain / application / infrastructure layers

Replace the history-driven lib/ + utils/ split with DDD layers:
- domain/: aggregate, repositories, value objects, pure parsers/format
- application/: feed-service, email-processor, feed-fetcher, stats
- infrastructure/: logging, auth, KV/R2 adapters, HTTP, framework glue

Pure file relocation; imports updated mechanically. Behaviour unchanged.

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

src/routes/favicon.ts

@@ -2,7 +2,10 @@ import { Context } from "hono";
 import { Env } from "../types";
 import { FeedRepository } from "../domain/feed-repository";
 import { FeedId } from "../domain/value-objects/feed-id";
-import { cacheFaviconForDomain, getCachedIcon } from "../utils/favicon-fetcher";
+import {
+  cacheFaviconForDomain,
+  getCachedIcon,
+} from "../infrastructure/favicon-fetcher";
 
 export const FAVICON_PATH = "/favicon.svg";