Rebuild: post a batch early when 100k files wait; log failing stats refreshes

A picture group's 500k articles made batches over the server's 256 MB
body limit: HTTP 413, the range given back and leased again in a loop
(148597: 39 times in 30 minutes). The worker now also posts once
100,000 completed or edge-cut files wait (RangeIndexer.pending).
A stats refresh that failed kept the previous totals silently; it
logs the error now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/edge/range_indexer.ts

@@ -188,6 +188,13 @@ export class RangeIndexer {
     return this.aggregator.openFiles
   }
 
+  /** Files completed and fragments cut since the last take: how big the next batch is. */
+  get pending(): number {
+    let files = this.batch.fragments.length
+    for (const list of this.completed.values()) files += list.length
+    return files
+  }
+
   /** Completed files, gathered per release until the batch is taken: one chunk per release per batch. */
   private completed = new Map<string, AggregatedFile<EdgeSegment>[]>()