feat(backend): add automatic DB migrations

Add a lightweight migration runner with schema_migrations tracking, run pending migrations during backend startup before the scheduler, and keep a manual backend-migrate entrypoint.

The change also moves the existing lockout and task-thread-ID schema steps into shared migration modules, updates docs, and archives the OpenSpec change.

tests/test_main_manager.py

@@ -56,6 +56,11 @@ class BackendManagerUvTests(unittest.TestCase):
         self.assertEqual(cmd[1:3], ["run", "python"])
         self.assertEqual(cmd[3:5], [str(main.REPO_ROOT / "main.py"), "backend"])
 
+    def test_backend_migrate_command_is_registered(self) -> None:
+        args = main.build_parser().parse_args(["backend-migrate"])
+
+        self.assertIs(args.func, main.run_backend_migrations)
+
 
 if __name__ == "__main__":
     unittest.main()