http: /skus/{sku_code}: raise 404 HTTPException if SKU is not found

We currently assert if the provided SKU number is not found, causing an
internal server error.  Raise an HTTPException with status 404 instead.

src/canadiantracker/http.py

@@ -116,7 +116,9 @@ async def one_sku(
     request: Request, sku_code: str
 ) -> starlette.templating._TemplateResponse:
     sku = _repository.get_sku_by_code(sku_code)
-    assert sku
+    if sku is None:
+        raise HTTPException(status_code=404, detail="SKU not found")
+
     product_url = sku.product.url
     if product_url:
         sku_url = make_sku_url(sku.code, product_url)