fix: dynamic back button URL for product detail page

- Pass back_url from routes to template
- Letzshop products go back to /admin/marketplace/letzshop
- Marketplace products go back to /admin/marketplace-products

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routes/admin_pages.py

@@ -731,6 +731,7 @@ async def admin_letzshop_product_detail_page(
             "request": request,
             "user": current_user,
             "product_id": product_id,
+            "back_url": "/admin/marketplace/letzshop",
         },
     )
 
@@ -782,6 +783,7 @@ async def admin_marketplace_product_detail_page(
             "request": request,
             "user": current_user,
             "product_id": product_id,
+            "back_url": "/admin/marketplace-products",
         },
     )