fix: add /healthz route to dashboard for Railway healthcheck

The dashboard Next.js app had no healthcheck endpoint, causing Railway
deployments to fail after build. Adds a simple GET /healthz route that
returns 200 and configures railway.toml to use it explicitly.

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

Author: broomva

dashboard/apps/web/app/healthz/route.ts

@@ -0,0 +1,5 @@
+import { NextResponse } from "next/server";
+
+export function GET() {
+  return NextResponse.json({ status: "ok" }, { status: 200 });
+}

dashboard/railway.toml

@@ -3,5 +3,7 @@ builder = "dockerfile"
 dockerfilePath = "Dockerfile"
 
 [deploy]
+healthcheckPath = "/healthz"
+healthcheckTimeout = 120
 restartPolicyType = "on_failure"
 restartPolicyMaxRetries = 5