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.

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();
   }
 });