Rebuild: the coordinator stops on SIGTERM

It waited for every chunk in flight, and a connection the server held
back (after killed coordinators left sessions open) waits up to the
10-minute reply timeout: it never seemed to stop. It now waits at most
15 s, then closes all connections, busy ones too; the greeting and login
time out after 30 s. And run() ends with terminate(), as edge:node does.

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

app/services/nntp_pool.ts

@@ -10,6 +10,7 @@ import { XoverClient } from '#lib/nntp/index'
  */
 class NntpPool {
   private idle: XoverClient[] = []
+  private busy = new Set<XoverClient>()
   private open = 0
   private waiting: ((client: XoverClient) => void)[] = []
 
@@ -19,6 +20,7 @@ class NntpPool {
 
   async use<T>(work: (client: XoverClient) => Promise<T>): Promise<T> {
     const client = await this.acquire()
+    this.busy.add(client)
     let broken = false
     try {
       return await work(client)
@@ -26,6 +28,7 @@ class NntpPool {
       broken = true
       throw error
     } finally {
+      this.busy.delete(client)
       this.release(client, broken)
     }
   }
@@ -71,8 +74,10 @@ class NntpPool {
     else this.idle.push(client)
   }
 
+  /** Closes every connection, also those in use: their commands fail. */
   async close() {
-    for (const client of this.idle.splice(0)) client.close()
+    for (const client of [...this.idle.splice(0), ...this.busy]) client.close()
+    this.busy.clear()
     this.open = 0
   }
 }