feat: Implement core backend server with file upload, monitoring, S2A API proxy, and team processing features.

frontend/src/pages/Upload.tsx

@@ -120,13 +120,10 @@ export default function Upload() {
 
       try {
         const text = await file.text()
-        const json = JSON.parse(text)
-        const accounts = Array.isArray(json) ? json : [json]
-
         const res = await fetch('/api/upload/validate', {
           method: 'POST',
           headers: { 'Content-Type': 'application/json' },
-          body: JSON.stringify({ accounts }),
+          body: JSON.stringify({ content: text, filename: file.name }),
         })
 
         const data = await res.json()