Add backend unit test framework and modularize settings UI

Introduced a Vitest-based backend unit testing framework with supporting scripts, helpers, and GitHub Actions integration. Refactored the admin settings page to a modular architecture, splitting monolithic logic into feature-specific tabs and hooks for improved maintainability and testability. Updated documentation to reflect the new testing setup and settings architecture, and added new dependencies for testing utilities.

tests/api/admin-metrics.routes.test.ts

@@ -0,0 +1,51 @@
+/**
+ * Component: Admin Metrics API Route Tests
+ * Documentation: documentation/testing.md
+ */
+
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { createPrismaMock } from '../helpers/prisma';
+
+let authRequest: any;
+
+const prismaMock = createPrismaMock();
+const requireAuthMock = vi.hoisted(() => vi.fn());
+const requireAdminMock = vi.hoisted(() => vi.fn());
+
+vi.mock('@/lib/db', () => ({
+  prisma: prismaMock,
+}));
+
+vi.mock('@/lib/middleware/auth', () => ({
+  requireAuth: requireAuthMock,
+  requireAdmin: requireAdminMock,
+}));
+
+describe('Admin metrics route', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    authRequest = { user: { id: 'admin-1', role: 'admin' } };
+    requireAuthMock.mockImplementation((_req: any, handler: any) => handler(authRequest));
+    requireAdminMock.mockImplementation((_req: any, handler: any) => handler());
+  });
+
+  it('returns metrics and system health', async () => {
+    prismaMock.request.count
+      .mockResolvedValueOnce(10)
+      .mockResolvedValueOnce(2)
+      .mockResolvedValueOnce(5)
+      .mockResolvedValueOnce(1)
+      .mockResolvedValueOnce(0);
+    prismaMock.user.count.mockResolvedValueOnce(3);
+    prismaMock.$queryRaw.mockResolvedValueOnce(1);
+
+    const { GET } = await import('@/app/api/admin/metrics/route');
+    const response = await GET({} as any);
+    const payload = await response.json();
+
+    expect(payload.totalRequests).toBe(10);
+    expect(payload.systemHealth.status).toBe('healthy');
+  });
+});
+
+