Refine cursor function typing

[sutyum]

Summary

• narrow CursorExecFunction to use sqlite3.Cursor and sqlite3.Connection
• import this alias in SQLite database implementation
• update accompanying type stub

Testing

• ruff format personal_graph/database/db.py personal_graph/database/sqlite/sqlite.py stub/personal_graph/database/sqlite/sqlite.pyi
• ruff check personal_graph/database/db.py personal_graph/database/sqlite/sqlite.py stub/personal_graph/database/sqlite/sqlite.pyi
• poetry run python -m mypy . (fails: No module named mypy)