feat(exam-prep F2): порт браузера вариантов + API /variants + POST /attempts + редирект /exam9

backend/src/server.js

@@ -366,9 +366,8 @@ app.get(['/exam-prep/:examKey/:view', '/exam-prep/:examKey/:view/*'], (req, res,
   sendExamPrep(res, file);
 });
 
-// NOTE: /exam9 remains live (served by static middleware as exam9.html) until F2
-// ports the variant browser into /exam-prep/:examKey/variants. The 301 redirect
-// will be added at that point.
+// Legacy /exam9 → new variants browser (F2 port complete)
+app.get('/exam9', (_req, res) => res.redirect(301, '/exam-prep/math9/variants'));
 
 // Serve HTML files without extension  (/dashboard → dashboard.html)
 // In dev: disable cache so edits are always picked up immediately