Close NNTP connections left idle for NNTP_IDLE_CLOSE_SECONDS

Eweka drops quiet connections, so the next command on one failed
("Spot inspection failed: NNTP connection closed"), and idle connections
held account slots that downloads could use. The coordinator pool now
closes free connections (and the plain one) on its sync once idle for
60 s; dry prober lanes, the spotter between rounds and the spot inspector
with nothing to inspect close theirs too, reconnecting when work comes.

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

app/services/prober.ts

@@ -121,6 +121,7 @@ export class Prober {
    */
   private async lane() {
     let client: XoverClient | null = null
+    let usedAt = Date.now()
     try {
       while (!this.stopped) {
         if (this.running > this.share.granted) break
@@ -134,6 +135,12 @@ export class Prober {
         this.stats.waitMs += Date.now() - waited
         this.dry = batch.length === 0
         if (batch.length === 0) {
+          // Nothing to probe for a while: the connection closes (nntpIdleCloseMs), and opens again with work.
+          const idleMs = coordinatorConfig.nntpIdleCloseMs
+          if (client && idleMs > 0 && Date.now() - usedAt >= idleMs) {
+            client.close()
+            client = null
+          }
           this.stats.idleMs += 30_000
           await sleep(30_000)
           continue
@@ -170,6 +177,7 @@ export class Prober {
             }
           }
           this.stats.inspectMs += Date.now() - inspected
+          usedAt = Date.now()
         } catch (error: any) {
           logger.warn(`Probing ${batch[0].messageId} and ${batch.length - 1} more: ${error.message}`)
           for (const candidate of batch) this.outcomes.push({ candidate, status: 'waiting' })