feat(app): Next.js 15 + Auth.js v5 portal skeleton

Lands the minimum surface so a developer can:

  cd platform/orca-platform && make dev-up
  cd platform/tenant-registry && make dev
  cd platform/portal && make install && make dev
  open http://acme.localhost:3000

and complete a real OIDC sign-in against the breakpilot-dev realm.

Layout:
  src/middleware.ts                host→slug URL rewrite; backstage carve-out
  src/auth.ts                      Auth.js v5 Keycloak provider; passes
                                   tenant_id/slug/org_roles/products/plan/status
                                   claims through to the session
  src/app/api/auth/[...nextauth]/  Auth.js handlers (GET, POST)
  src/app/layout.tsx               root html shell
  src/app/page.tsx                 apex landing
  src/app/[slug]/layout.tsx        fetches tenant via lib/tenant-registry
  src/app/[slug]/page.tsx          redirect to /dashboard
  src/app/[slug]/dashboard/page.tsx
                                   signed-out → Sign in with Keycloak
                                   signed-in  → welcome + Sign out
  src/lib/host.ts                  testable host parser (apex/tenant/backstage)
  src/lib/tenant-registry.ts       fetch client for the Go service

Tooling:
  vitest                           13 tests, 100% coverage of src/lib/
  Next.js 15 build                 compiles all routes; output: standalone
  ESLint flat config               next/core-web-vitals + next/typescript

Real RBAC enforcement, the rest of the customer-area surfaces, and the
backstage shell land per the M5.2 / M10.1 schedule. This is just enough
to be the first thing a developer codes in.

Refs: M5.1 (skeleton)

src/lib/tenant-registry.ts

@@ -0,0 +1,29 @@
+// Tenant Registry client — fetches tenant data from the Go service.
+// Skeleton-mode: read-only by-slug lookup. The portal middleware uses this
+// to resolve `<slug>.localhost:3000` → tenant context before rendering.
+
+export type Tenant = {
+  id: string;
+  slug: string;
+  name: string;
+  status: "active" | "trial" | "frozen" | "archived" | "demo";
+  plan: "starter" | "professional" | "enterprise";
+  products: string[];
+  created_at: string;
+};
+
+function baseUrl(): string {
+  return process.env.TENANT_REGISTRY_URL ?? "http://localhost:8080";
+}
+
+export async function fetchTenantBySlug(slug: string): Promise<Tenant | null> {
+  const res = await fetch(`${baseUrl()}/v1/tenants/by-slug/${encodeURIComponent(slug)}`, {
+    headers: { accept: "application/json" },
+    cache: "no-store",
+  });
+  if (res.status === 404) return null;
+  if (!res.ok) {
+    throw new Error(`tenant-registry: ${res.status} ${res.statusText}`);
+  }
+  return (await res.json()) as Tenant;
+}