refactor: strengthen upload boundary types

app/api/upload/git/route.ts

@@ -6,6 +6,7 @@ import { classifyFileChanges } from '@/lib/diff-files'
 import { getModelFolderPath } from '@/lib/model-paths'
 import { cleanupStagingUpload, ensurePreparedStagingAssets, readStagedManifest } from '@/lib/upload-staging'
 import { acquireUploadLock, releaseUploadLock } from '@/lib/upload-lock'
+import { parseStagingRequestBody } from '@/lib/upload-request'
 
 export const runtime = 'nodejs'
 export const dynamic = 'force-dynamic'
@@ -23,11 +24,8 @@ export async function POST(req: NextRequest) {
   let stagingId: string
 
   try {
-    const body = await req.json()
-    stagingId = body.stagingId
-    if (!stagingId || typeof stagingId !== 'string') {
-      throw new Error('stagingId manquant')
-    }
+    const body: unknown = await req.json()
+    stagingId = parseStagingRequestBody(body).stagingId
     const manifest = await readStagedManifest(stagingId)
     folderName = manifest.folderName
   } catch (err) {