Run /force downloads at low CPU + idle IO priority (nice/ionice)

Wrap the forced yt-dlp download (and its node/ffmpeg children) and the rsync
upload with 'nice -n 19 ionice -c 3' so the known-big, slow /force job yields the
CPU and disk to everything else under load while still finishing fast when idle.
Normal short downloads stay at full priority. Best-effort: prefix is empty if the
tools are absent.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bot.py

@@ -61,6 +61,14 @@ _SSH_OPTS = ["-p", FORCE_SSH_PORT, "-o", "BatchMode=yes", "-o", "StrictHostKeyCh
              *(["-i", FORCE_SSH_KEY] if FORCE_SSH_KEY else [])]
 _RSYNC_RSH = "ssh " + " ".join(shlex.quote(o) for o in _SSH_OPTS)
 
+# /force is known to be big and slow, so run its subprocesses (yt-dlp + the node
+# n-sig solver and ffmpeg merge it spawns, plus rsync) at lowest CPU and idle IO
+# priority. They yield to everything else under load but still run full-speed when
+# the box is idle; child processes inherit the niceness. Normal short downloads
+# are left at full priority for responsiveness.
+_NICE = (["nice", "-n", "19"] if shutil.which("nice") else []) + \
+        (["ionice", "-c", "3"] if shutil.which("ionice") else [])
+
 logging.basicConfig(
     level=logging.INFO,
     format="%(asctime)s [%(levelname)s] %(message)s",
@@ -649,6 +657,7 @@ def _force_download_and_publish(url: str, token: str) -> tuple[str | None, int |
         tokdir = os.path.join(stage, token)
         os.makedirs(tokdir)
         cmd = [
+            *_NICE,
             YTDLP, "--no-playlist", "--no-warnings", "--js-runtimes", "node",
             "-f", "best[ext=mp4]/best", "--merge-output-format", "mp4",
             "--max-filesize", FORCE_MAX_FILESIZE,
@@ -674,7 +683,7 @@ def _force_download_and_publish(url: str, token: str) -> tuple[str | None, int |
         size_mb = os.path.getsize(os.path.join(tokdir, fname)) // (1024 * 1024)
 
         up = subprocess.run(
-            ["rsync", "-a", "--chmod=F644,D755", "-e", _RSYNC_RSH,
+            [*_NICE, "rsync", "-a", "--chmod=F644,D755", "-e", _RSYNC_RSH,
              tokdir, f"{FORCE_REMOTE}:{FORCE_REMOTE_DIR}/"],
             capture_output=True, text=True, timeout=RSYNC_TIMEOUT,
         )