test(auth-registration-status): pin all four discovery branches

The route is read-only and its envelope shape (`registrationEnabled`
boolean) gates iOS sign-in + web onboarding. Four branches walked:
  1. singleton row exists with registrationEnabled: true → reports true
  2. singleton row exists with registrationEnabled: false → reports
     false (admin-disabled tenant, invite-only)
  3. singleton row missing → reports schema default true (fresh self-
     hosted instance, no-config baseline)
  4. findUnique throws → reports false (fail-closed; critical defense
     against a DB blip silently unlocking sign-up on a tenant that
     disabled registration)

No code change; the route's current behaviour is correct. Pin it
before someone refactors fail-closed → fail-open by accident.

src/__tests__/api/auth/registration-status/route.test.ts

@@ -0,0 +1,116 @@
+/**
+ * v1.4.40 W-INSIGHTS SB-7 — pin the four discovery branches of
+ * `GET /api/auth/registration-status`.
+ *
+ * The route is read-only; the iOS sign-in flow + the web onboarding
+ * dispatcher both depend on its envelope shape (`registrationEnabled`
+ * boolean) staying byte-stable. A regression that flipped the
+ * fail-closed branch to fail-open would silently allow registration on
+ * a self-hosted deployment whose admin set `registrationEnabled: false`
+ * and an upstream DB blip lost the row.
+ *
+ * Four branches walked:
+ *
+ *   1. **Singleton row exists with `registrationEnabled: true`** →
+ *      envelope reports `true`.
+ *   2. **Singleton row exists with `registrationEnabled: false`** →
+ *      envelope reports `false`. (Admin-disabled tenant — invite-only.)
+ *   3. **Singleton row missing (`findUnique` returns `null`)** →
+ *      envelope reports the schema default `true` (open registration —
+ *      the no-config baseline for a fresh self-hosted instance).
+ *   4. **`findUnique` throws** (DB outage / connection drop) → envelope
+ *      fails closed with `false`. Critical: a DB blip on a tenant
+ *      that has disabled registration must NOT unlock the sign-up
+ *      flow.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    appSettings: {
+      findUnique: vi.fn(),
+    },
+  },
+}));
+
+vi.mock("@/lib/auth/audit", () => ({
+  auditLog: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/db-compat", () => ({
+  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/logging/transports", () => ({ emitIfSampled: vi.fn() }));
+
+vi.mock("next/headers", () => ({
+  headers: vi.fn(async () => ({ get: () => null })),
+  cookies: vi.fn(async () => ({
+    get: () => undefined,
+    set: () => {},
+    delete: () => {},
+  })),
+}));
+
+import { GET } from "@/app/api/auth/registration-status/route";
+import { prisma } from "@/lib/db";
+
+const callGet = GET as unknown as (...args: never[]) => Promise<Response>;
+
+interface RegistrationStatusBody {
+  data: { registrationEnabled: boolean };
+}
+
+beforeEach(() => {
+  vi.resetAllMocks();
+});
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("GET /api/auth/registration-status", () => {
+  it("reports `true` when the singleton row carries registrationEnabled: true", async () => {
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue({
+      id: "singleton",
+      registrationEnabled: true,
+    } as never);
+
+    const res = await callGet();
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as RegistrationStatusBody;
+    expect(body.data.registrationEnabled).toBe(true);
+  });
+
+  it("reports `false` when the singleton row carries registrationEnabled: false (admin-disabled tenant)", async () => {
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue({
+      id: "singleton",
+      registrationEnabled: false,
+    } as never);
+
+    const res = await callGet();
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as RegistrationStatusBody;
+    expect(body.data.registrationEnabled).toBe(false);
+  });
+
+  it("falls back to the schema default `true` when the singleton row is missing (fresh self-hosted instance)", async () => {
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue(null as never);
+
+    const res = await callGet();
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as RegistrationStatusBody;
+    expect(body.data.registrationEnabled).toBe(true);
+  });
+
+  it("fails closed with `false` when the DB lookup throws (defense-in-depth against silent unlock)", async () => {
+    vi.mocked(prisma.appSettings.findUnique).mockRejectedValue(
+      new Error("connection terminated"),
+    );
+
+    const res = await callGet();
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as RegistrationStatusBody;
+    expect(body.data.registrationEnabled).toBe(false);
+  });
+});