feat(unsubscribe): RFC 8058 one-click unsubscribe on feed deletion

Capture each sender's List-Unsubscribe one-click URL during ingestion
(stored per sender in feed metadata, mirroring the iconDomain pattern) and
fire one-click POSTs via ctx.waitUntil when a feed is deleted, so newsletters
stop mailing the now-dead address. Tracked with a new unsubscribes_sent
counter surfaced on the status page and /api/stats.

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

src/utils/stats.ts

@@ -8,6 +8,7 @@ const EMPTY_COUNTERS: Counters = {
   feeds_deleted: 0,
   emails_received: 0,
   emails_rejected: 0,
+  unsubscribes_sent: 0,
 };
 
 export async function getCounters(kv: KVNamespace): Promise<Counters> {
@@ -39,6 +40,7 @@ export async function bumpCounters(
     current.feeds_deleted += changes.feeds_deleted ?? 0;
     current.emails_received += changes.emails_received ?? 0;
     current.emails_rejected += changes.emails_rejected ?? 0;
+    current.unsubscribes_sent += changes.unsubscribes_sent ?? 0;
     if (changes.last_email_at) current.last_email_at = changes.last_email_at;
     if (changes.last_feed_created_at)
       current.last_feed_created_at = changes.last_feed_created_at;