Fix ffmpeg overwrite and empty language fallback

- Add -y flag to ffmpeg to overwrite mkstemp-created empty temp files
  (ffmpeg exits 0 without writing when file exists and stdin is /dev/null)
- Fix language fallback: use `or` instead of dict.get default so empty
  string from client falls back to config value
- Fix language value in config: ru -> russian

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/audio/processor.py

@@ -59,6 +59,7 @@ class AudioProcessor:
             "ffmpeg",
             "-hide_banner",
             "-loglevel", "warning",
+            "-y",
             "-i", input_path,
             "-ar", f"{audio_rate}",
             "-ac", "1",  # Монофонический звук