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/application/feed-service.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect } from "vitest";
 import { createMockEnv } from "../test/setup";
 import { createFeedRecord, editFeed } from "./feed-service";
+import { getCounters } from "./stats";
 import type { Env } from "../types";
 
 const mkEnv = (overrides: Partial<Env> = {}) =>
@@ -42,6 +43,14 @@ describe("createFeedRecord — TTL policy", () => {
     // 1h (server) wins over 9999h (client).
     expect(config.expires_at!).toBeLessThan(before + TWO_HOURS);
   });
+
+  it("bumps the feeds_created counter via the FeedCreated domain event", async () => {
+    const env = mkEnv();
+    await createFeedRecord(env, { ...baseInput });
+    const counters = await getCounters(env.EMAIL_STORAGE);
+    expect(counters.feeds_created).toBe(1);
+    expect(counters.last_feed_created_at).toBeDefined();
+  });
 });
 
 describe("editFeed — TTL policy", () => {