refactor: move KV repositories to infrastructure (Track P — points 2, 6c)

Make the domain stop depending on infrastructure ("imports point inward").

- Point 2: relocate the four KV adapters (FeedRepository, IconRepository,
  WebSubSubscriptionRepository, CountersRepository) from domain/ to
  infrastructure/, where the logger import is legitimate. The domain now keeps
  only the pure key schema (feed-keys.ts), the Feed aggregate and value objects;
  it imports nothing outward. Deliberately no hand-rolled 24-method port
  interface (YAGNI without DI) — relocation alone fixes the direction.
- Point 6c: EmailParser.extractFeedId now returns a validated FeedId value
  object instead of a raw string, so the most untrusted input (an inbound
  recipient address) is guarded at the parse boundary and no longer round-trips
  through FeedId.fromTrusted in the ingest path.

All import paths updated; CLAUDE.md source layout/KV-schema notes updated.
351 tests pass; tsc --noEmit clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/infrastructure/counters-repository.ts

@@ -0,0 +1,23 @@
+import { Counters, Env } from "../types";
+import { STATS_KEY } from "../domain/feed-keys";
+
+/**
+ * KV access for the monitoring counters singleton (`stats:counters`). The
+ * increment policy lives in the application layer (utils/stats.ts); this
+ * repository owns only the raw read/write of the blob.
+ */
+export class CountersRepository {
+  constructor(private readonly kv: KVNamespace) {}
+
+  static from(env: Env): CountersRepository {
+    return new CountersRepository(env.EMAIL_STORAGE);
+  }
+
+  async getRaw(): Promise<Counters | null> {
+    return (await this.kv.get(STATS_KEY, { type: "json" })) as Counters | null;
+  }
+
+  async put(counters: Counters): Promise<void> {
+    await this.kv.put(STATS_KEY, JSON.stringify(counters));
+  }
+}