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 <[email protected]>

src/lib/email-processor.ts

@@ -112,6 +112,16 @@ export async function validateEmail(
       response: new Response("Feed does not exist", { status: 404 }),
     };
   }
+  if (
+    feedConfig.expires_at !== undefined &&
+    feedConfig.expires_at <= Date.now()
+  ) {
+    logger.warn("Rejected email: feed expired", { feedId });
+    return {
+      ok: false,
+      response: new Response("Feed has expired", { status: 410 }),
+    };
+  }
 
   const allowedSenders = (feedConfig.allowed_senders || [])
     .map(normalizeEmail)