feat(stats): count emails forwarded to the catch-all fallback

Adds an emails_forwarded counter (a subset of emails_rejected) bumped on a
successful FALLBACK_FORWARD_ADDRESS forward. Dropped = rejected − forwarded.
Surfaced in the /api/v1/stats response and the public status page.

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

src/infrastructure/counters-repository.test.ts

@@ -14,6 +14,7 @@ describe("CountersRepository", () => {
       feeds_deleted: 0,
       emails_received: 2,
       emails_rejected: 0,
+      emails_forwarded: 0,
       unsubscribes_sent: 0,
     });
     expect(await repo.getRaw()).toMatchObject({ emails_received: 2 });