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/attachments.ts

@@ -0,0 +1,9 @@
+import { Env } from "../types";
+
+// Returns the attachment bucket only when the feature is enabled, so callers can
+// narrow cleanly. Attachments are on whenever R2 is bound, unless explicitly
+// turned off with ATTACHMENTS_ENABLED="false".
+export function getAttachmentBucket(env: Env): R2Bucket | undefined {
+  if (env.ATTACHMENTS_ENABLED === "false") return undefined;
+  return env.ATTACHMENT_BUCKET;
+}