M5-T09: add exposed_entities table and ExposableEntity provider framework

tests/test_modbus_models.py

@@ -144,7 +144,13 @@ def test_modbus_reading_recorded_at_index_exists(modbus_db):
 
 
 def test_downgrade_removes_modbus_tables(tmp_path: Path):
-    """downgrade -1 from head must drop both modbus tables cleanly."""
+    """Downgrading past the modbus migration must drop both modbus tables cleanly.
+
+    We downgrade to the explicit revision ``20260621_08_totp`` (the down_revision
+    of the modbus tables migration), so the test stays correct as new revisions
+    are added on top.  Using an explicit target rather than ``-2`` avoids drift
+    with chain growth (same fix pattern as T02 applied to the TOTP downgrade test).
+    """
     db_path = tmp_path / "downgrade_modbus_test.db"
     db_url = f"sqlite:///{db_path}"
     alembic_cfg = _make_app_alembic_config(db_url)
@@ -157,8 +163,11 @@ def test_downgrade_removes_modbus_tables(tmp_path: Path):
     assert "modbus_reading" in inspector.get_table_names()
     engine.dispose()
 
-    # Downgrade one step removes the modbus migration.
-    command.downgrade(alembic_cfg, "-1")
+    # Downgrade to the explicit down_revision of the modbus tables migration
+    # ("20260621_08_totp"), consistent with how T02 fixed the TOTP downgrade test.
+    # Using an explicit target instead of "-2" ensures the test stays correct even
+    # as new revisions are added on top of this one (no drift with chain growth).
+    command.downgrade(alembic_cfg, "20260621_08_totp")
 
     engine = create_engine(db_url, connect_args={"check_same_thread": False})
     inspector = inspect(engine)