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/processors/retry-missing-torrents.processor.test.ts

@@ -0,0 +1,45 @@
+/**
+ * Component: Retry Missing Torrents Processor Tests
+ * Documentation: documentation/backend/services/scheduler.md
+ */
+
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { createPrismaMock } from '../helpers/prisma';
+import { createJobQueueMock } from '../helpers/job-queue';
+
+const prismaMock = createPrismaMock();
+const jobQueueMock = createJobQueueMock();
+
+vi.mock('@/lib/db', () => ({
+  prisma: prismaMock,
+}));
+
+vi.mock('@/lib/services/job-queue.service', () => ({
+  getJobQueueService: () => jobQueueMock,
+}));
+
+describe('processRetryMissingTorrents', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('queues search jobs for awaiting_search requests', async () => {
+    prismaMock.request.findMany.mockResolvedValue([
+      {
+        id: 'req-1',
+        audiobook: { id: 'a1', title: 'Book', author: 'Author', audibleAsin: 'ASIN1' },
+      },
+    ]);
+
+    const { processRetryMissingTorrents } = await import('@/lib/processors/retry-missing-torrents.processor');
+    const result = await processRetryMissingTorrents({ jobId: 'job-1' });
+
+    expect(result.success).toBe(true);
+    expect(jobQueueMock.addSearchJob).toHaveBeenCalledWith(
+      'req-1',
+      expect.objectContaining({ id: 'a1', title: 'Book', author: 'Author' })
+    );
+  });
+});
+
+