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 <noreply@anthropic.com>

src/types/index.ts

@@ -89,6 +89,9 @@ export interface Counters {
   feeds_deleted: number;
   emails_received: number;
   emails_rejected: number;
+  // Subset of emails_rejected: non-feed mail forwarded to FALLBACK_FORWARD_ADDRESS
+  // instead of dropped. Dropped count = emails_rejected − emails_forwarded.
+  emails_forwarded: number;
   unsubscribes_sent: number;
   last_email_at?: string; // ISO 8601
   last_feed_created_at?: string; // ISO 8601