fix(srs): keep directional review cards visible

backend/services/vocabService.js

@@ -123,10 +123,13 @@ export default class VocabService {
     return crypto.createHash('sha1').update(raw).digest('hex');
   }
 
-  _buildSrsPairFingerprint({ courseId, learning, reference, direction = 'BOTH' }) {
+  _buildSrsPairFingerprint({ courseId, learning, reference }) {
+    // The repetition plan is course-wide and trains both directions. Older
+    // rows were created with L2R/R2L while newly extracted course vocabulary
+    // uses BOTH. Direction must therefore not split one vocabulary pair into
+    // separate identities or make a valid old card disappear from the plan.
     return [
       Number(courseId) || 0,
-      String(direction || 'BOTH').toUpperCase(),
       this._normalizeSrsText(learning),
       this._normalizeSrsText(reference)
     ].join('|');