Rebuild: prefetch report samples what the connections do

Once a second: pool connections with a command in flight, loops
fetching/writing/idle; per request GROUP time, XOVER time to the status
line and the transfer, transfers with a silence over 5 s. The old
"connections busy" (fetch time of the chunks finished in the minute)
said 59-75% while all 40 connections were in flight all the time:
dropped.

Homelab: 39-40 of 40 in flight, no idle loops; the status line comes at
once, the whole 8-35 s is the transfer, with data flowing (few gaps
over 5 s): ~3 MB per chunk at 0.1-0.4 MB/s per connection.

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

app/services/nntp_pool.ts

@@ -32,6 +32,11 @@ class NntpPool {
     return this.acquire().then((client) => this.run(client, work))
   }
 
+  /** Connections with a command in flight (the prefetcher's, the planner's). */
+  get busyCount() {
+    return this.busy.size
+  }
+
   /** How long ago the connection was opened, in ms. */
   ageOf(client: XoverClient): number {
     return Date.now() - (this.openedAt.get(client) ?? Date.now())