refactor: introduce domain events for feed side effects (Track E — point 5)

Light "collect + dispatch" variant: the Feed aggregate records FeedEvents
(FeedCreated, EmailIngested) on the mutations that have consequences, exposed via
pullEvents(). A new application dispatcher (feed-events.applyFeedEvents) maps
those events to their side effects — counters (awaited) plus WebSub pings and
favicon fetches handed to a BackgroundScheduler. This removes the inline,
scattered side effects from the ingest hot path (email-processor) and from
createFeedRecord; the aggregate is now the source of truth for "what happened".

Side effects with no aggregate mutation (rejected email, feed deletion bypassing
the aggregate, bulk admin ops, the cron, unsubscribes-sent) stay imperative by
design — there is no aggregate event for them to ride on.

BackgroundScheduler type moved to infrastructure/worker.ts (shared). CLAUDE.md
updated. 355 tests pass (+4 event tests); tsc --noEmit clean.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/application/feed-events.ts

@@ -0,0 +1,42 @@
+import { Env } from "../types";
+import { FeedEvent } from "../domain/events";
+import { FeedId } from "../domain/value-objects/feed-id";
+import { BackgroundScheduler } from "../infrastructure/worker";
+import { bumpCounters } from "./stats";
+import { notifySubscribers } from "../infrastructure/websub";
+import { cacheFaviconForDomain } from "../infrastructure/favicon-fetcher";
+
+/**
+ * Apply the side effects of a feed's domain events — the single place that maps
+ * "what happened" (FeedCreated, EmailIngested) to its consequences. Counter
+ * writes are awaited (they must land); WebSub pings and favicon fetches are
+ * handed to the caller's background scheduler (`ctx.waitUntil` at the edge, a
+ * no-op when none is available).
+ */
+export async function applyFeedEvents(
+  feedId: FeedId,
+  events: FeedEvent[],
+  env: Env,
+  schedule: BackgroundScheduler,
+): Promise<void> {
+  for (const event of events) {
+    switch (event.type) {
+      case "FeedCreated":
+        await bumpCounters(env.EMAIL_STORAGE, {
+          feeds_created: 1,
+          last_feed_created_at: new Date().toISOString(),
+        });
+        break;
+      case "EmailIngested":
+        await bumpCounters(env.EMAIL_STORAGE, {
+          emails_received: 1,
+          last_email_at: new Date().toISOString(),
+        });
+        schedule(notifySubscribers(feedId.value, env));
+        if (event.iconDomain) {
+          schedule(cacheFaviconForDomain(event.iconDomain, env));
+        }
+        break;
+    }
+  }
+}