fix: specify WAV format for atomic save temp file

soundfile cannot infer format from .tmp extension, causing all
generations to fail with 'No format specified and unable to get
format from file extension'

backend/utils/audio.py

@@ -92,8 +92,9 @@ def save_audio(
         # Ensure parent directory exists
         Path(path).parent.mkdir(parents=True, exist_ok=True)
 
-        # Write to temporary file first
-        sf.write(temp_path, audio, sample_rate)
+        # Write to temporary file first (explicit format since .tmp
+        # extension is not recognised by soundfile)
+        sf.write(temp_path, audio, sample_rate, format='WAV')
 
         # Atomic rename to final path
         os.replace(temp_path, path)