Fix loss of context in POST handler.

This caused the principal to not be available when creating the new
audit record.

src/pages.go

···
 		result := UpdateFromRepository(ctx, webRoot, repoURL, auth.branch)
 		resultChan <- result
 		observeSiteUpdate("webhook", &result)
-	}(context.Background())
+	}(r.Context())
 
 	var result UpdateResult
 	select {