refactor: move emitToSession broadcast into ws-server.broadcastToSession

Consolidates WS-first + SSE-fallback + guest forwarding logic
into ws-server.js so classroomController doesn't duplicate delivery.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/controllers/classroomController.js

@@ -25,22 +25,10 @@ const GUEST_EVENTS = new Set([
 ]);
 
 /* ── Helper: broadcast to all session participants ─────────────────────── */
+/* Delivery: WS-first (ws-server.js) with SSE fallback for non-WS users.   */
 function emitToSession(sessionId, data) {
-  const session = db.prepare('SELECT class_id, teacher_id FROM classroom_sessions WHERE id=?').get(sessionId);
-  if (!session) return;
-
-  if (session.class_id) {
-    emitToClass(session.class_id, data);
-    emit(session.teacher_id, data); // teacher is not in class_members — emit separately
-  } else {
-    // personal session — emit to teacher + each invited user
-    emit(session.teacher_id, data);
-    const invites = db.prepare('SELECT user_id FROM classroom_invites WHERE session_id=?').all(sessionId);
-    for (const { user_id } of invites) emit(user_id, data);
-  }
-
-  // Forward whitelisted events to guest viewers
-  if (GUEST_EVENTS.has(data.type)) emitToGuests(sessionId, data);
+  // ws-server handles WS-first + SSE-fallback + guest forwarding internally
+  require('../ws-server').broadcastToSession(sessionId, data, GUEST_EVENTS.has(data.type));
 }
 
 /* ── Helper: check if user has access to session ──────────────────────── */