tighten backend regression coverage

Add targeted tests for rejection paths, settlement overrides, and key analytics flows, and align localization/research docs with the shipped contract.

backend/tests/localization_contract.rs

@@ -0,0 +1,28 @@
+use std::collections::BTreeSet;
+
+use serde_json as json;
+
+#[test]
+fn english_and_swedish_localization_bundles_share_the_same_keys() {
+    let en: json::Value =
+        json::from_str(include_str!("../../contracts/localization/en.json")).unwrap();
+    let sv: json::Value =
+        json::from_str(include_str!("../../contracts/localization/sv.json")).unwrap();
+
+    let en_keys: BTreeSet<_> = en.as_object().unwrap().keys().cloned().collect();
+    let sv_keys: BTreeSet<_> = sv.as_object().unwrap().keys().cloned().collect();
+
+    assert_eq!(en_keys, sv_keys);
+    assert!(!en_keys.is_empty());
+
+    for key in &en_keys {
+        assert!(
+            !en[key].as_str().unwrap().trim().is_empty(),
+            "missing English text for {key}"
+        );
+        assert!(
+            !sv[key].as_str().unwrap().trim().is_empty(),
+            "missing Swedish text for {key}"
+        );
+    }
+}