feat(lib): report each finished upload's rate to the server

The speed other users see for us in `user` lookups and in the server's
own ranking comes from SendUploadSpeed (code 121), which the client
never sent, so it read zero however much we served. Each completed
upload now reports its measured bytes per second, as Nicotine+ and
slskd do.

soulseek-rs-lib/tests/e2e.rs

@@ -1769,6 +1769,22 @@ fn two_real_clients_search_and_download() {
         "a finished upload sets the advertised speed"
     );
 
+    // The server hears about it too (SendUploadSpeed), which is where every
+    // other client's `user` lookup and the server's own ranking read it.
+    let deadline = Instant::now() + Duration::from_secs(10);
+    let mut recorded = 0;
+    while Instant::now() < deadline && recorded == 0 {
+        leecher
+            .request_user_info("e2e_sharer")
+            .expect("ask about the sharer");
+        std::thread::sleep(Duration::from_millis(250));
+        recorded = leecher
+            .user_info("e2e_sharer")
+            .and_then(|info| info.stats)
+            .map_or(0, |stats| stats.average_speed);
+    }
+    assert!(recorded > 0, "the server should record the upload's speed");
+
     let _ = std::fs::remove_dir_all(share_dir);
     let _ = std::fs::remove_dir_all(download_dir);
 }