tests: isolate the database so the suite can never touch the real DB

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.)

tests/conftest.py

@@ -4,6 +4,29 @@ Creates a minimal Flask+SocketIO app that replicates the relevant
 endpoints and event handlers without importing the full web_server.py
 (which would try to initialize Spotify, Soulseek, Plex, etc.)."""
 
+# ── TEST-DATABASE ISOLATION — MUST run before any other import ─────────────
+# Force every MusicDatabase()/get_database() call in the suite onto a throwaway
+# temp database so a test can NEVER open or write the real
+# database/music_library.db. MusicDatabase resolves its default path from
+# os.environ['DATABASE_PATH'] (see database/music_database.py); setting it here,
+# at conftest import (before any test module loads), redirects ALL default-path
+# DB access to /tmp.
+#
+# Why this is non-negotiable: tests exercise modules (e.g. album_mbid_cache)
+# that call get_database() with no path → the real DB. Running those writers
+# against the live DB over a WSL-mounted Windows drive corrupted a user's
+# library. This guarantees it can't recur — tests get their own disposable DB.
+import os as _os
+import tempfile as _tempfile
+import atexit as _atexit
+import shutil as _shutil
+
+if not _os.environ.get('SOULSYNC_TEST_DB_READY'):
+    _TEST_DB_DIR = _tempfile.mkdtemp(prefix='soulsync-testdb-')
+    _os.environ['DATABASE_PATH'] = _os.path.join(_TEST_DB_DIR, 'test_music_library.db')
+    _os.environ['SOULSYNC_TEST_DB_READY'] = '1'
+    _atexit.register(lambda: _shutil.rmtree(_TEST_DB_DIR, ignore_errors=True))
+
 import copy
 import pytest
 import threading

tests/test_db_isolation_guard.py

@@ -0,0 +1,32 @@
+"""Guard: the test suite must NEVER resolve the real database/music_library.db.
+
+Tests exercise modules that call get_database() with no path. If that resolves
+to the live DB, test writes can corrupt a real library (this happened, over a
+WSL-mounted Windows drive). conftest.py sets DATABASE_PATH to a temp file before
+anything imports; these tests prove it sticks for every default-path access.
+"""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+_REAL = os.path.join('database', 'music_library.db')
+
+
+def test_database_path_env_is_isolated():
+    p = os.environ.get('DATABASE_PATH', '')
+    assert 'soulsync-testdb-' in p, f"DATABASE_PATH not isolated: {p!r}"
+
+
+def test_musicdatabase_default_path_never_real():
+    from database.music_database import MusicDatabase
+    resolved = str(Path(MusicDatabase().database_path).resolve())
+    assert 'soulsync-testdb-' in resolved, resolved
+    assert not resolved.replace('\\', '/').endswith('database/music_library.db'), resolved
+
+
+def test_get_database_path_never_real():
+    from database.music_database import get_database
+    resolved = str(Path(get_database().database_path).resolve())
+    assert 'soulsync-testdb-' in resolved, resolved