Add batch upload endpoint to avoid sequential upload+OCR bottleneck

Client sends all files in a single request to /api/upload/batch,
server receives them all upfront via multer then processes OCR
sequentially. Eliminates network round-trips between each file.

server/src/routes/upload.ts

@@ -149,6 +149,74 @@ router.post('/', upload.single('document'), async (req: AuthenticatedRequest, re
   }
 });
 
+const MAX_BATCH_FILES = 20;
+
+router.post('/batch', upload.array('documents', MAX_BATCH_FILES), async (req: AuthenticatedRequest, res) => {
+  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 {
+    const files = req.files as Express.Multer.File[];
+    if (!files || files.length === 0) {
+      activeUploads--;
+      return res.status(400).json({ error: 'No files uploaded' });
+    }
+
+    const useOcr = req.body?.useOcr === 'true' || req.body?.useOcr === true;
+    if (useOcr) {
+      const groups = req.user?.groups || [];
+      const hasGroupAccess = groups.includes('kaboot-ai-access');
+      const status = req.user ? getSubscriptionStatus(req.user.sub) : null;
+      const hasSubscriptionAccess = status?.status === 'active';
+
+      if (!hasGroupAccess && !hasSubscriptionAccess) {
+        return res.status(403).json({ error: 'OCR is available to Pro subscribers only.' });
+      }
+    }
+
+    const results = [];
+    for (const file of files) {
+      const normalizedMime = normalizeMimeType(file.mimetype, file.originalname);
+      const processed = await processDocument(file.buffer, normalizedMime, { useOcr });
+
+      if (processed.type === 'native') {
+        results.push({
+          type: 'native',
+          content: (processed.content as Buffer).toString('base64'),
+          mimeType: processed.mimeType,
+          originalName: file.originalname
+        });
+      } else {
+        results.push({
+          type: 'text',
+          content: processed.content as string,
+          mimeType: processed.mimeType,
+          originalName: file.originalname
+        });
+      }
+    }
+
+    res.json(results);
+  } catch (error) {
+    console.error('Batch upload processing error:', error);
+    res.status(500).json({
+      error: error instanceof Error ? error.message : 'Failed to process documents'
+    });
+  } finally {
+    activeUploads--;
+    drainQueue();
+  }
+});
+
 router.use((err: Error, _req: any, res: any, _next: any) => {
   if (err instanceof multer.MulterError) {
     if (err.code === 'LIMIT_FILE_SIZE') {