Several tests exercise modules (e.g. album_mbid_cache) that call get_database()
with no path, which resolves to the real database/music_library.db. Running
those writers against the live DB over a WSL-mounted Windows drive corrupted a
user's library. conftest never isolated the DB path.
conftest.py now sets os.environ['DATABASE_PATH'] to a throwaway temp file at
import time (before any test module loads). MusicDatabase resolves its default
path from DATABASE_PATH, so EVERY default-path MusicDatabase()/get_database()
call in the suite now lands in /tmp — the real DB is never opened. Temp dir is
removed at exit.
Guard tests assert DATABASE_PATH, MusicDatabase().database_path, and
get_database().database_path all resolve into the temp dir and never to
database/music_library.db. (album_mbid_cache tests, which were failing against
the corrupted live DB, now pass clean on the isolated temp DB.)
