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 <noreply@anthropic.com>

src/domain/events.ts

@@ -0,0 +1,16 @@
+/**
+ * Domain events the Feed aggregate records when it mutates. They describe *what
+ * happened* in business terms; the application layer decides which side effects
+ * to run (counters, WebSub pings, favicon caching) via a dispatcher. This keeps
+ * the aggregate ignorant of infrastructure and the orchestration code free of
+ * scattered, inline side effects.
+ *
+ * Only mutations that currently have side effects emit events — feed creation
+ * and email ingestion. Edits and removals carry no side effect, so they emit
+ * nothing. Side effects that don't flow through the aggregate (a rejected email,
+ * a feed deletion that bypasses the aggregate, bulk admin operations) stay
+ * outside this mechanism by design — they have no aggregate event to ride on.
+ */
+export type FeedEvent =
+  | { type: "FeedCreated" }
+  | { type: "EmailIngested"; iconDomain?: string };