fix(vocab): reject lexical hints paired with sentences

backend/migrations-active/20260827000000-remove-invalid-karon-srs-card.cjs

@@ -0,0 +1,17 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface) {
+    // A legacy gap-fill hint was persisted as a vocabulary answer. The pair
+    // "jetzt/heute" → "Karon naa ko diri" is not a valid translation card.
+    await queryInterface.sequelize.query(`
+      DELETE FROM community.vocab_srs_item
+      WHERE lower(trim(learning)) = 'jetzt/heute'
+        AND lower(trim(reference)) = 'karon naa ko diri';
+    `);
+  },
+
+  async down() {
+    // Do not restore an invalid study card.
+  }
+};

backend/services/vocabService.js

@@ -201,8 +201,15 @@ export default class VocabService {
     const rightLooksSentence = rightWords >= 3 || /[?!.].+\S/.test(rightText);
     const leftLooksShortFragment = leftWords <= 1 && leftText.length <= 12;
     const rightLooksShortFragment = rightWords <= 1 && rightText.length <= 12;
+    // A slash-separated hint such as "jetzt/heute" is a lexical alternative,
+    // never the translation of a full sentence such as "Karon naa ko diri".
+    const leftLooksLexicalHint = /\//.test(leftText) && leftWords <= 2 && leftText.length <= 24;
+    const rightLooksLexicalHint = /\//.test(rightText) && rightWords <= 2 && rightText.length <= 24;
 
-    return (leftLooksShortFragment && rightLooksSentence) || (rightLooksShortFragment && leftLooksSentence);
+    return (leftLooksShortFragment && rightLooksSentence)
+      || (rightLooksShortFragment && leftLooksSentence)
+      || (leftLooksLexicalHint && rightLooksSentence)
+      || (rightLooksLexicalHint && leftLooksSentence);
   }
 
   _calculateSrsSchedule(item, { correct } = {}) {