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>

config.json

@@ -1,7 +1,7 @@
 {
     "service_port": 5042,
     "model_path": "/home/text-generation/models/whisper/podlodka-turbo",
-    "language": "ru",
+    "language": "russian",
     "enable_history": true,
     "max_history_days": 30,
     "chunk_length_s": 28,