chore: upgrade SQLAlchemy to 2.0 and bump all stale deps ⬆️

- SQLAlchemy 1.4 → 2.0.46: migrate all session.query() calls to
  select() / session.execute() style; move declarative_base import
  from ext.declarative to sqlalchemy.orm; explicit conn.commit()
  after DDL in _migrate(); drop sessionmaker(bind=) keyword
- persist-queue 1.0 → 1.1, pymysql 1.1.1 → 1.1.2,
  dnspython 2.7 → 2.8, pyyaml 6.0.2 → 6.0.3
- pytest 8.3 → 9.0.2, pytest-cov 6.1 → 7.0,
  pytest-mock 3.14 → 3.15.1, black 25.1 → 26.1

97 tests pass, zero deprecation warnings

tests/conftest.py

@@ -21,7 +21,7 @@ def engine():
 
 @pytest.fixture
 def db_session(engine):
-    session = sessionmaker(bind=engine)()
+    session = sessionmaker(engine)()
     yield session
     session.close()