Queue concurrent uploads instead of rejecting with 503

Server: replace hard 503 rejection with a bounded queue (50 slots,
2min timeout) so requests wait for a processing slot instead of failing.

Client: upload files sequentially instead of via Promise.all, and
retry with exponential backoff on 503/504 responses.

hooks/useGame.ts

@@ -730,29 +730,41 @@ export const useGame = (defaultGameConfig?: GameConfig) => {
     };
   }, []);
 
-  const uploadDocument = async (file: File, useOcr: boolean = false): Promise<ProcessedDocument> => {
+  const uploadDocument = async (file: File, useOcr: boolean = false, maxRetries = 5): Promise<ProcessedDocument> => {
     if (!auth.user?.access_token) {
       throw new Error('Authentication required to upload documents');
     }
     
-    const formData = new FormData();
-    formData.append('document', file);
-    formData.append('useOcr', String(useOcr));
-    
-    const response = await fetch(`${BACKEND_URL}/api/upload`, {
-      method: 'POST',
-      headers: {
-        'Authorization': `Bearer ${auth.user.access_token}`,
-      },
-      body: formData
-    });
-    
-    if (!response.ok) {
-      const error = await response.json();
-      throw new Error(error.error || 'Failed to upload document');
+    for (let attempt = 0; attempt <= maxRetries; attempt++) {
+      const formData = new FormData();
+      formData.append('document', file);
+      formData.append('useOcr', String(useOcr));
+      
+      const response = await fetch(`${BACKEND_URL}/api/upload`, {
+        method: 'POST',
+        headers: {
+          'Authorization': `Bearer ${auth.user.access_token}`,
+        },
+        body: formData
+      });
+      
+      if (response.status === 503 || response.status === 504) {
+        if (attempt < maxRetries) {
+          const delay = Math.min(1000 * 2 ** attempt, 15000);
+          await new Promise(r => setTimeout(r, delay));
+          continue;
+        }
+      }
+      
+      if (!response.ok) {
+        const error = await response.json();
+        throw new Error(error.error || 'Failed to upload document');
+      }
+      
+      return response.json();
     }
     
-    return response.json();
+    throw new Error('Failed to upload document after multiple retries');
   };
 
   const startQuizGen = async (options: { 
@@ -773,9 +785,10 @@ export const useGame = (defaultGameConfig?: GameConfig) => {
       
       let documents: ProcessedDocument[] | undefined;
       if (options.files && options.files.length > 0) {
-        documents = await Promise.all(
-          options.files.map(file => uploadDocument(file, options.useOcr))
-        );
+        documents = [];
+        for (const file of options.files) {
+          documents.push(await uploadDocument(file, options.useOcr));
+        }
       }
       
       const generateOptions: GenerateQuizOptions = {

server/src/routes/upload.ts

@@ -44,6 +44,31 @@ function tieredUploadLimiter(req: AuthenticatedRequest, res: Response, next: Nex
 
 let activeUploads = 0;
 const MAX_CONCURRENT_UPLOADS = 5;
+const MAX_QUEUE_SIZE = 50;
+const QUEUE_TIMEOUT_MS = 120_000; // 2 minutes
+
+const waitForSlot = (): Promise<void> => {
+  if (activeUploads < MAX_CONCURRENT_UPLOADS) return Promise.resolve();
+  return new Promise((resolve, reject) => {
+    let wrappedResolve: () => void;
+    const timeout = setTimeout(() => {
+      const idx = uploadQueue.indexOf(wrappedResolve);
+      if (idx !== -1) uploadQueue.splice(idx, 1);
+      reject(new Error('Upload queue timeout'));
+    }, QUEUE_TIMEOUT_MS);
+    wrappedResolve = () => { clearTimeout(timeout); resolve(); };
+    uploadQueue.push(wrappedResolve);
+  });
+};
+
+const uploadQueue: Array<() => void> = [];
+
+const drainQueue = () => {
+  while (uploadQueue.length > 0 && activeUploads < MAX_CONCURRENT_UPLOADS) {
+    const next = uploadQueue.shift();
+    if (next) next();
+  }
+};
 
 router.use(requireAuth);
 router.use(tieredUploadLimiter);
@@ -66,10 +91,16 @@ const upload = multer({
 });
 
 router.post('/', upload.single('document'), async (req: AuthenticatedRequest, res) => {
-  if (activeUploads >= MAX_CONCURRENT_UPLOADS) {
+  if (activeUploads >= MAX_CONCURRENT_UPLOADS && uploadQueue.length >= MAX_QUEUE_SIZE) {
     return res.status(503).json({ error: 'Server busy processing uploads. Please try again shortly.' });
   }
 
+  try {
+    await waitForSlot();
+  } catch {
+    return res.status(504).json({ error: 'Upload timed out waiting in queue. Please try again.' });
+  }
+
   activeUploads++;
 
   try {
@@ -114,6 +145,7 @@ router.post('/', upload.single('document'), async (req: AuthenticatedRequest, re
     });
   } finally {
     activeUploads--;
+    drainQueue();
   }
 });