fix: add completed/interrupted guard in session upsert (out-of-order event defense)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/modules/learning-session/learning-session.repository.ts

@@ -80,6 +80,11 @@ export class LearningSessionRepository {
     });
 
     if (existing) {
+      // Guard: don't update completed/interrupted sessions (defense against out-of-order events)
+      if (existing.status === 'completed' || existing.status === 'interrupted') {
+        return existing;
+      }
+
       const update: any = {
         lastEventAt: new Date(Number(data.timestampMs)),
       };