feat(foundry): redirect to signin page on auth API errors

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

Author: NathanFlurry

foundry/packages/backend/src/services/better-auth.ts

@@ -549,6 +549,9 @@ export function initBetterAuthService(actorClient: any, options: { apiUrl: strin
         strategy: "compact",
       },
     },
+    onAPIError: {
+      errorURL: stripTrailingSlash(options.appUrl) + "/signin",
+    },
     socialProviders: {
       github: {
         clientId: requireEnv("GITHUB_CLIENT_ID"),