fix: make insights saving safer

[olalie]

## Summary
Make insights saving safer and prevent problems caused by multiple TapMap instances running at the same time.

Changes:
- Save `insights.json` atomically
- Fall back safely if insights data is corrupt or invalid
- Keep only recognised insights sections
- Add a simple PID lock file to prevent concurrent writers
- Handle stale lock files safely
- Add tests for insights loading, saving and lock handling
- Add `Flask` explicitly to `requirements.txt`
- Remove unused `tomli` dependency

## Testing
- Verified locally on Windows
- Verified stale lock replacement
- Verified second instance exits cleanly
- Verified insights save/load roundtrip
- `pytest` → 173 passed