(feature) add queued backup worker foundation

Move backup execution out of the management command into a reusable
backup runner service that can execute an existing BackupRun record.

Add queue primitives and a run_pobsync_worker command so manual backup
requests can be recorded as queued SQL state and processed outside the
web request path.

Add a worker Docker service and pobsync worker CLI alias, with tests for
queued run creation, worker execution, manual run typing, and command
mapping.

src/pobsync_backend/tests/test_console_entrypoint.py

@@ -46,3 +46,10 @@ class ConsoleEntrypointTests(SimpleTestCase):
 
         self.assertEqual(exit_code, 0)
         execute.assert_called_once_with(["pobsync", "discover_pobsync_snapshots", "--host", "web-01"])
+
+    def test_maps_worker_alias_to_django_command(self) -> None:
+        with patch("pobsync.cli.execute_from_command_line") as execute:
+            exit_code = main(["worker", "--once"])
+
+        self.assertEqual(exit_code, 0)
+        execute.assert_called_once_with(["pobsync", "run_pobsync_worker", "--once"])