Add storage backend support and fix CI

[schapman1974]

Summary

• add optional storage backend support and related tests
• keep TinyDB JSON as the default storage backend
• fix CI lint failures in tests and tinymongo imports
• update docs to remove fork wording and correct backend/default storage notes
• update setup.cfg metadata key to avoid setuptools deprecation

Verification

• .venv/bin/python -m pytest -q
• .venv/bin/python -m ruff check .
• .venv/bin/python -m build
• .venv/bin/python -m twine check dist/*