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 <[email protected]>

src/routes/admin.test.ts

@@ -3,7 +3,7 @@ import { http, HttpResponse } from "msw";
 import { Hono } from "hono";
 import app from "./admin";
 import { createMockEnv, server } from "../test/setup";
-import { getCounters } from "../utils/stats";
+import { getCounters } from "../application/stats";
 import { Env } from "../types";
 
 describe("Admin Routes", () => {