Day page: a past date with no activity is not found, and never written to disk

Probing any old date returned an empty page with a 200 and left a junk file
behind for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

daysnap.js

@@ -90,7 +90,13 @@ function get(key) {
     if (key > today) return null;
   }
   const d = compute(key);
-  if (d && key !== today) save(d);
+  if (!d) return null;
+  if (key !== today) {
+    // a past day with nothing in it is either before the contract or beyond the index's reach:
+    // say not found rather than serving an empty page, and never write a file for it
+    if (d.totals.payouts + d.totals.joins === 0) return null;
+    save(d);
+  }
   return d;
 }
 function save(d) { try { fs.writeFileSync(dayFile(d.key), JSON.stringify(d)); } catch (e) {} }