feat: implement upsert functionality in fetch_history.py to accumulate OI/funding data

- Added `--upsert` flag to `fetch_history.py` for merging new data into existing parquet files.
- Implemented `upsert_parquet()` function to update existing rows with new values where `oi_change` and `funding_rate` are 0.0, while appending new rows.
- Created tests in `tests/test_fetch_history.py` to validate upsert behavior.
- Updated `.gitignore` to include `.cursor/` directory.

Made-with: Cursor

.gitignore

@@ -9,4 +9,5 @@ venv/
 models/*.pkl
 data/*.parquet
 .worktrees/
-.DS_Store
+.DS_Store
+.cursor/