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/files.ts

@@ -1,6 +1,6 @@
 import { Context } from "hono";
 import { Env } from "../types";
-import { getAttachmentBucket } from "../utils/attachments";
+import { getAttachmentBucket } from "../infrastructure/attachments";
 
 export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
   const bucket = getAttachmentBucket(c.env);