fix: accept /collab WebSocket upgrade without trailing slash

The Hocuspocus provider connects to /collab (no trailing slash),
but the upgrade handler only matched /collab/. Also add debug
logging for incoming WS messages and proper error handling.

Made-with: Cursor

backend/src/server.ts

@@ -150,12 +150,24 @@ app.get("/api/auth/status", async (req, res) => {
 const wss = new WebSocketServer({ noServer: true });
 
 httpServer.on("upgrade", (req, socket, head) => {
-  if (req.url?.startsWith("/collab/")) {
+  const url = req.url || "";
+  // Hocuspocus v3 provider connects to /collab (no document name in path)
+  if (url === "/collab" || url.startsWith("/collab/") || url.startsWith("/collab?")) {
+    console.log(`[ws] upgrade request for ${url}`);
     wss.handleUpgrade(req, socket, head, (ws) => {
+      ws.setMaxListeners(Infinity);
+      ws.on("error", (error) => {
+        console.error("[ws] socket error:", error.message);
+      });
+      ws.on("message", (data: Buffer, isBinary: boolean) => {
+        const buf = Buffer.isBuffer(data) ? data : Buffer.from(data as any);
+        console.log(`[ws-debug] msg ${buf.length}B binary=${isBinary} first20=${buf.slice(0, 20).toString("hex")}`);
+      });
       const token = extractToken(req.headers.cookie);
       hocuspocus.handleConnection(ws, req, { token });
     });
   } else {
+    console.log(`[ws] rejected upgrade for ${url}`);
     socket.destroy();
   }
 });