fix: Add file upload endpoint and fix frontend upload path

frontend/src/components/quality/character-panel.tsx

@@ -70,11 +70,12 @@ export function CharacterPanel({ storyboardId }: CharacterPanelProps) {
         if (!file) return
 
         try {
-            // Upload to server
+            // Upload to server - use quality API endpoint
             const formData = new FormData()
             formData.append('file', file)
 
-            const response = await fetch(`/api/upload?storyboard_id=${storyboardId}&type=character`, {
+            const apiBase = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000/api'
+            const response = await fetch(`${apiBase}/quality/upload?storyboard_id=${storyboardId}&type=character`, {
                 method: 'POST',
                 body: formData,
             })