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 <noreply@anthropic.com>

src/routes/home.tsx

@@ -119,6 +119,7 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
               tone="danger"
             />
             <Stat label="Net feeds" value={netFeeds} />
+            <Stat label="Unsubscribes sent" value={stats.unsubscribes_sent} />
             <Stat
               label="Last feed created"
               value={formatRelative(stats.last_feed_created_at)}