hotfix: import Header from fastapi — caused RUNTIME_ERROR on 24349be

backend/main.py uses Header(default=None, alias="X-Admin-Password")
in the new POST /api/admin/upload/reextract endpoint I just added,
but Header wasn't on the `from fastapi import …` line. Python
raised NameError at module-import time, which surfaced as the HF
Space stage=RUNTIME_ERROR for 24349be.

Added Header to the fastapi imports.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/main.py

@@ -18,7 +18,7 @@ import uuid
 from pathlib import Path
 from typing import Optional
 
-from fastapi import FastAPI, File, Form, HTTPException, Request, UploadFile
+from fastapi import FastAPI, File, Form, Header, HTTPException, Request, UploadFile
 from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse, JSONResponse