refactor: govern background job lifecycle

tests/test_job_runner.py

@@ -68,6 +68,29 @@ class JobRunnerTests(unittest.TestCase):
         )
         self.assertEqual(self.repository.recent(1)[0]["status"], "failed")
 
+    def test_scheduler_start_is_idempotent_and_stop_waits_for_exit(self) -> None:
+        first = self.runner.start_scheduler(
+            lambda: None, interval_seconds=60, initial_delay_seconds=60
+        )
+        second = self.runner.start_scheduler(
+            lambda: None, interval_seconds=60, initial_delay_seconds=60
+        )
+        self.assertIs(first, second)
+        self.assertTrue(first.is_alive())
+        self.assertTrue(self.runner.stop_scheduler())
+        self.assertFalse(first.is_alive())
+
+    def test_scheduler_can_restart_after_an_orderly_stop(self) -> None:
+        first = self.runner.start_scheduler(
+            lambda: None, interval_seconds=60, initial_delay_seconds=60
+        )
+        self.assertTrue(self.runner.stop_scheduler())
+        second = self.runner.start_scheduler(
+            lambda: None, interval_seconds=60, initial_delay_seconds=60
+        )
+        self.assertIsNot(first, second)
+        self.assertTrue(self.runner.stop_scheduler())
+
 
 if __name__ == "__main__":
     unittest.main()