Refactor logging, clean dead code, fix temp files and async tasks

- Replace f-string logging with % formatting throughout
- Fix create_temp_file: use mkstemp() instead of mkdtemp() (no leaked dirs)
- Add cleanup background thread and copy.deepcopy to AsyncTaskManager
- Fix transcribe_audio_async signature to accept transcription_service + params
- Use resample_poly instead of scipy_resample for better audio quality
- Remove unused validate_file/BinaryIO methods from FileValidator
- Add requests to requirements.txt
- Add error handling for pip install in server.sh
- Add __all__ and module docstrings to __init__.py files

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

requirements.txt

@@ -17,3 +17,4 @@ scipy==1.13.1
 transformers==4.49.0
 accelerate==1.4.0
 python-magic==0.4.27
+requests==2.32.3