fix: add missing func import in product delete endpoint

NameError on `func.count()` when checking orphaned CadFile references.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/routers/products.py

@@ -11,7 +11,7 @@ from pathlib import Path
 from fastapi import APIRouter, Depends, File, HTTPException, Query, UploadFile, status
 from fastapi.responses import StreamingResponse
 from pydantic import BaseModel
-from sqlalchemy import select, or_, text
+from sqlalchemy import select, or_, text, func
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload, joinedload