Fix UnboundLocalError in cleanup function

The cleanup() callback was trying to delete generator_worker and
out_writer_worker unconditionally, but these variables are only
defined in certain code paths. This caused UnboundLocalError when
cleanup was called after requests that didn't create these workers.

Wrap the deletions in try/except blocks to handle cases where the
variables weren't created.

speech.py

@@ -734,8 +734,15 @@ async def generate_speech(request: GenerateSpeechRequest):
 
         def cleanup():
             ffmpeg_proc.kill()
-            del generator_worker
-            del out_writer_worker
+            # Only delete workers if they were created
+            try:
+                del generator_worker
+            except NameError:
+                pass
+            try:
+                del out_writer_worker
+            except NameError:
+                pass
 
         return StreamingResponse(content=ffmpeg_proc.stdout, media_type=media_type, background=cleanup)
     # Use Silero for tts-1-silero