fix(db): 重用 DatabaseManager engine pool

tests/test_database_manager_cache.py

@@ -0,0 +1,16 @@
+from database.manager import DatabaseManager
+
+
+def test_database_manager_reuses_engine_for_same_sqlite_path(tmp_path):
+    DatabaseManager._instance_cache.clear()
+    db_url = f"sqlite:///{tmp_path / 'cache.db'}"
+
+    first = DatabaseManager(db_url)
+    second = DatabaseManager(db_url)
+
+    try:
+        assert second.engine is first.engine
+        assert second.Session is first.Session
+    finally:
+        DatabaseManager._instance_cache.clear()
+        first.engine.dispose()