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/infrastructure/worker.ts

@@ -0,0 +1,10 @@
+import { Context } from "hono";
+
+/** Calls ctx.waitUntil() without throwing when the ExecutionContext is absent (e.g. Node tests). */
+export function waitUntilSafe(c: Context, promise: Promise<unknown>): void {
+  try {
+    c.executionCtx.waitUntil(promise);
+  } catch {
+    // ExecutionContext unavailable in Node test environment — ignore.
+  }
+}