feat: add /admin/letzshop/products/{id} route for product details

- Add new route that mirrors the letzshop orders pattern
- Update product links in letzshop-products-tab.html to use new route

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

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

app/routes/admin_pages.py

@@ -710,6 +710,31 @@ async def admin_letzshop_order_detail_page(
     )
 
 
+@router.get(
+    "/letzshop/products/{product_id}",
+    response_class=HTMLResponse,
+    include_in_schema=False,
+)
+async def admin_letzshop_product_detail_page(
+    request: Request,
+    product_id: int = Path(..., description="Marketplace Product ID"),
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render Letzshop product detail page.
+    Shows full product information from the marketplace.
+    """
+    return templates.TemplateResponse(
+        "admin/marketplace-product-detail.html",
+        {
+            "request": request,
+            "user": current_user,
+            "product_id": product_id,
+        },
+    )
+
+
 # ============================================================================
 # PRODUCT CATALOG ROUTES
 # ============================================================================