Fix AI Coach typo, fix bulk register UI, and update exam tiers

backend/main.py

@@ -1,7 +1,7 @@
 from fastapi import FastAPI, Depends, HTTPException, status, Request, UploadFile, File, Form, BackgroundTasks
 from fastapi.responses import StreamingResponse
 from fastapi.middleware.cors import CORSMiddleware
-from sqlmodel import Session, select
+from sqlmodel import Session, select, delete
 from contextlib import asynccontextmanager
 from typing import List, Dict, Any, Optional
 from uuid import UUID
@@ -286,6 +286,21 @@ async def bulk_register_students(file: UploadFile = File(...), session: Session
     session.commit()
     return results
 
+class DeleteUsersRequest(BaseModel):
+    user_ids: List[UUID]
+
+@app.delete("/api/admin/students/bulk", tags=["Admin"])
+def bulk_delete_students(payload: DeleteUsersRequest, session: Session = Depends(get_session)):
+    for uid in payload.user_ids:
+        user = session.get(models.User, uid)
+        if user:
+            # Delete associated records manually since foreign key cascade might not be configured in sqlmodel by default
+            session.exec(delete(models.Attempt).where(models.Attempt.user_id == uid))
+            session.exec(delete(models.Mistake).where(models.Mistake.user_id == uid))
+            session.delete(user)
+    session.commit()
+    return {"success": True}
+
 @app.post("/api/auth/register", tags=["Authentication"])
 def register(payload: RegisterPayload, session: Session = Depends(get_session)):
     import bcrypt