docs(files): complete German reference coverage

tests/test_documentation_contract.py

@@ -0,0 +1,7 @@
+from govoplan_files.backend.manifest import get_manifest
+
+
+def test_static_documentation_has_complete_german_reference_copy() -> None:
+    for topic in get_manifest().documentation:
+        german = topic.translations.get("de", {})
+        assert all(german.get(field, "").strip() for field in ("title", "summary", "body")), topic.id