Cherry-pick test suite from tests branch into dev

[Copilot]

The tests branch contained a complete database-layer test suite (108 tests) and CI workflow that had not been merged into dev. This brings those commits forward.

Changes

• Test suite (matchypatchy_package/tests/): 108 pytest tests covering database/setup.py, database/mpdb.py, database/media.py, and database/location.py
• conftest.py: Stubs out PyQt6, cv2, and related modules at collection time so DB-layer tests run headless without a display server; includes _VersionMock to handle PyQt6 version integer comparisons; provides tmp_db and populated_db fixtures
• pyproject.toml: Added [tool.pytest.ini_options] with testpaths = ["tests"]
• CI workflow (.github/workflows/tests.yml): Runs pytest with pytest-cov on push/PR to main; uploads coverage to Codecov
• 3 source bug fixes in location.py, media.py, and mpdb.py that were caught while writing the tests

Note: mpdb.py had a cherry-pick conflict — the tests branch was adding thread-local chroma/collection properties that already exist on dev from a later PR. HEAD was kept.