fix(auth): survive a stale session whose user was deleted

A JWT session pointing at a user no longer in the DB (e.g. after a DB reset)
made getUserSettings create settings for a non-existent user -> FK violation
(P2003) -> Server Component render crash. getUserSettings now returns defaults
on P2003; the (app) layout detects the missing user and redirects to a new
server-side /logout route that clears the cookie, avoiding the middleware
redirect loop that otherwise blocks reaching /login.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/(app)/layout.tsx

@@ -1,7 +1,23 @@
+import { redirect } from "next/navigation";
+import { auth } from "@/lib/auth";
+import { prisma } from "@/lib/prisma";
 import { Sidebar } from "@/components/layout/sidebar";
 import { Header } from "@/components/layout/header";
 
-export default function AppLayout({ children }: { children: React.ReactNode }) {
+export default async function AppLayout({ children }: { children: React.ReactNode }) {
+  // Guard against a stale JWT session whose user no longer exists in the
+  // database (e.g. after a DB reset). The signed cookie still passes edge
+  // middleware, but every downstream query keyed on session.user.id would fail.
+  // Send such sessions to /logout, which clears the cookie and returns to login.
+  const session = await auth();
+  if (session?.user?.id) {
+    const user = await prisma.user.findUnique({
+      where: { id: session.user.id },
+      select: { id: true },
+    });
+    if (!user) redirect("/logout");
+  }
+
   return (
     <div className="flex h-screen overflow-hidden">
       <div className="hidden lg:block">

src/app/logout/route.ts

@@ -0,0 +1,9 @@
+import { signOut } from "@/lib/auth";
+
+// Server-side sign-out that clears the JWT session cookie and redirects to the
+// login page. Used to recover from a stale session whose user no longer exists
+// in the database (e.g. after a DB reset), which a client-only signOut can't
+// reach because the app crashes before rendering the user menu.
+export async function GET() {
+  await signOut({ redirectTo: "/login" });
+}