fix: bug audit — broken test imports, training data overlap, cleanup

- Fix test_utils.py importing build_annotation_json_path from main
  instead of core.annotations (all 59 tests pass now)
- Fix get_training_data double-counting clips at same start_time
  in both positive and soft sets — subtract positive from soft
- Add cancel_flag to train_classifier so training can be interrupted
  between videos (TrainWorker passes self as cancel_flag)
- Remove orphaned core/export.py (was for deleted server API)
- Remove stale Dockerfile and docker-compose.yml (referenced server)
- Clean up leftover server/__pycache__ and client/ build artifacts
- Add torch to requirements.txt (was only mentioned in comments)

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

main.py

@@ -350,6 +350,7 @@ class TrainWorker(QThread):
                 self._video_infos,
                 model_path=self._model_path,
                 embed_model=self._embed_model,
+                cancel_flag=self,
             )
             if self._cancel:
                 return