feat(app): Next.js 16 + Auth.js v5 portal skeleton

Next.js 16 + Auth.js v5 skeleton: host→slug middleware, tenant-context layout, OIDC sign-in flow against breakpilot-dev realm. 100% coverage on src/lib. Bumps next to 16.2.6 to clear trivy CVEs in 15.0.3.

src/app/[slug]/dashboard/page.tsx

@@ -0,0 +1,44 @@
+import { auth, signIn, signOut } from "@/auth";
+
+export default async function Dashboard({
+  params,
+}: {
+  params: Promise<{ slug: string }>;
+}) {
+  const { slug } = await params;
+  const session = await auth();
+
+  if (!session) {
+    async function login() {
+      "use server";
+      await signIn("keycloak", { redirectTo: `/${slug}/dashboard` });
+    }
+    return (
+      <div>
+        <h1>Sign in to {slug}</h1>
+        <form action={login}>
+          <button type="submit">Sign in with Keycloak</button>
+        </form>
+      </div>
+    );
+  }
+
+  async function logout() {
+    "use server";
+    await signOut({ redirectTo: `/${slug}/dashboard` });
+  }
+
+  return (
+    <div>
+      <h1>Dashboard</h1>
+      <p>
+        Welcome, {session.user?.name ?? session.user?.email ?? "user"}. This is the{" "}
+        <code>{slug}</code> dashboard. Real product tiles, settings, billing — land
+        in M5.2 / M10.1.
+      </p>
+      <form action={logout} style={{ marginTop: 24 }}>
+        <button type="submit">Sign out</button>
+      </form>
+    </div>
+  );
+}

src/app/[slug]/layout.tsx

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+import { notFound } from "next/navigation";
+import type { ReactNode } from "react";
+import { fetchTenantBySlug } from "@/lib/tenant-registry";
+
+export default async function TenantLayout({
+  children,
+  params,
+}: {
+  children: ReactNode;
+  params: Promise<{ slug: string }>;
+}) {
+  const { slug } = await params;
+  const tenant = await fetchTenantBySlug(slug);
+  if (!tenant) notFound();
+
+  return (
+    <div>
+      <header
+        style={{
+          padding: "12px 24px",
+          borderBottom: "1px solid #eaeaea",
+          background: "white",
+          display: "flex",
+          alignItems: "center",
+          justifyContent: "space-between",
+        }}
+      >
+        <strong>{tenant.name}</strong>
+        <span style={{ fontSize: 12, color: "#666" }}>
+          {tenant.plan} · {tenant.status}
+        </span>
+      </header>
+      <main style={{ padding: 24 }}>{children}</main>
+    </div>
+  );
+}

src/app/[slug]/page.tsx

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+import { redirect } from "next/navigation";
+
+export default async function TenantRoot({
+  params,
+}: {
+  params: Promise<{ slug: string }>;
+}) {
+  const { slug } = await params;
+  redirect(`/${slug}/dashboard`);
+}