feat(feeds): add configurable per-feed lifetime (TTL)

Replace the demo nightly KV wipe with a per-feed expiry. Feeds can be
given a lifetime at creation (and edited later); FEED_TTL_HOURS locks the
value server-side and greys out the UI field. Expired feeds stay visible
in admin (greyed, actions disabled), return 410 on rss/atom/entries, and
reject inbound emails. The scheduled handler now purges only expired
feeds (KV + R2 attachments) on an hourly global cron.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/atom.ts

@@ -15,6 +15,12 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
     if (!feedData) {
       return new Response("Feed not found", { status: 404 });
     }
+    if (
+      feedData.feedConfig.expires_at !== undefined &&
+      feedData.feedConfig.expires_at <= Date.now()
+    ) {
+      return new Response("Feed has expired", { status: 410 });
+    }
 
     const base = baseUrl(c.env);
     const selfUrl = new URL(c.req.url).origin + `/atom/${feedId}`;