fix: yt-dlp im temporären Verzeichnis ausführen (Streudatei „3“)

yt-dlp ruft zur Prüfung „ffmpeg -bsfs“ auf; das System-ffmpeg 6.1.1
(Ubuntu) legt dabei eine leere Datei „3“ im Arbeitsverzeichnis an.
yt-dlp läuft jetzt in einem temporären Verzeichnis mit absolutem
Zielpfad, die Datei verschwindet mit dem Verzeichnis.

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

yt_untertitel.py

@@ -6,16 +6,26 @@ import argparse
 import json
 import shutil
 import subprocess
+import tempfile
 
 YT_DLP = "yt-dlp"
 
 
+def run_yt_dlp(arguments):
+    """Run yt-dlp in a temporary working directory.
+
+    yt-dlp calls "ffmpeg -bsfs" to check ffmpeg; some ffmpeg builds (Ubuntu
+    6.1.1) then leave an empty file "3" in the working directory.
+    """
+    with tempfile.TemporaryDirectory() as tmp:
+        return subprocess.run(
+            [YT_DLP, *arguments], capture_output=True, text=True, cwd=tmp
+        )
+
+
 def fetch_info(url):
     """Return the video metadata from yt-dlp, without downloading anything."""
-    result = subprocess.run(
-        [YT_DLP, "--dump-single-json", "--skip-download", "--no-playlist", url],
-        capture_output=True, text=True,
-    )
+    result = run_yt_dlp(["--dump-single-json", "--skip-download", "--no-playlist", url])
     if result.returncode != 0:
         raise SystemExit(f"yt-dlp konnte das Video nicht abrufen:\n{result.stderr.strip()}")
     return json.loads(result.stdout)
@@ -70,14 +80,14 @@ def no_track_message(info, lang):
 
 def download(url, track, is_auto, target_base):
     """Download one subtitle track; yt-dlp writes <target_base>.<track>.vtt."""
-    command = [
-        YT_DLP, "--skip-download", "--no-playlist", "--force-overwrites",
+    result = run_yt_dlp([
+        "--skip-download", "--no-playlist", "--force-overwrites",
         "--write-auto-subs" if is_auto else "--write-subs",
         "--sub-langs", track, "--sub-format", "vtt",
-        "-o", f"{target_base}.%(ext)s",
+        # Absolute, because yt-dlp runs in a temporary directory
+        "-o", f"{Path(target_base).resolve()}.%(ext)s",
         url,
-    ]
-    result = subprocess.run(command, capture_output=True, text=True)
+    ])
     if result.returncode != 0:
         raise SystemExit(f"Download fehlgeschlagen:\n{result.stderr.strip()}")