docs(postbox): complete German reference coverage

tests/test_interface_documentation_contract.py

@@ -10,6 +10,17 @@ REPOSITORY_ROOT = Path(__file__).resolve().parents[1]
 
 
 class PostboxInterfaceDocumentationContractTests(unittest.TestCase):
+    def test_public_topics_have_complete_german_coverage(self) -> None:
+        topics = manifest.documentation
+
+        self.assertEqual(9, len(topics))
+        for topic in topics:
+            translation = topic.translations["de"]
+            self.assertEqual({"title", "summary", "body"}, set(translation))
+            self.assertTrue(
+                all(str(translation[field]).strip() for field in translation)
+            )
+
     def test_route_and_contributed_surfaces_remain_declared(self) -> None:
         frontend = manifest.frontend
         self.assertIsNotNone(frontend)