fix(tests): join background scan/enrich workers before closing the DB (#735)

Root cause of the flaky pytest segfault (exit 139): the background scan and enrichment daemon threads (_scan_runner/_enrich_runner) use the shared MetadataDB connection, but test fixtures closed that connection in teardown without stopping them. A daemon thread mid-query on a freed SQLite conn is a native use-after-free → SIGSEGV. The app's startup kicks a scan, so almost any app-booting fixture was vulnerable. It only surfaced now because got-feedback/feedBack#728 added a push trigger, so ci/test runs on every push to main.

Fix: server.py retains the scan/enrich thread handles and adds _join_background_db_threads(); every test fixture now joins the workers before conn.close(). Verified: the full suite runs to completion (no segfault) where it previously crashed at ~25%.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_settings_export_library_db.py

@@ -31,6 +31,7 @@ def server_mod(tmp_path, monkeypatch):
     yield mod
     conn = getattr(getattr(mod, "meta_db", None), "conn", None)
     if conn is not None:
+        getattr(__import__("sys").modules.get("server"), "_join_background_db_threads", lambda: None)()
         conn.close()
 
 
@@ -87,6 +88,7 @@ def test_export_includes_consistent_library_db_snapshot(client, server_mod, tmp_
             "SELECT title FROM songs WHERE filename = ?", ("snap.archive",)
         ).fetchall()
     finally:
+        getattr(__import__("sys").modules.get("server"), "_join_background_db_threads", lambda: None)()
         conn.close()
     assert rows == [("SnapSong",)]
 
@@ -271,6 +273,7 @@ def test_full_db_backup_restore_round_trip(client, server_mod, tmp_path):
             "SELECT title FROM songs WHERE filename = ?", ("keepme.archive",)
         ).fetchall()
     finally:
+        getattr(__import__("sys").modules.get("server"), "_join_background_db_threads", lambda: None)()
         conn.close()
     assert rows == [("KeepMe",)]
     assert not (tmp_path / "web_library.db.restore").exists()