Add rootless Podman fixes, and others

improve container startup for rootless Podman, plus related refactors and tests. Key changes:

- Add/modify Audiobookshelf-related code and wiring (src/lib/services/audiobookshelf/api.ts, library service refs) and update documentation TABLEOFCONTENTS to reference ABS implementation.
- Detect user namespace in docker/unified app-start.sh and redis-start.sh and skip gosu when running in rootless Podman to preserve UID mapping; improve startup logging and verification.
- Add utility/service files (auth-token-cache.service.ts, credential-migration.service.ts, cleanup-helpers.ts) and corresponding tests; update chapter-merger and metadata-tagger utilities/tests.
- Update many admin/auth API routes and tests to reflect changes in settings and integrations.
- Remove large AI agent and Audiobookshelf implementation guide docs (AGENTS.md and the implementation guide) and add README note about AI-assisted workflow.

These changes enable Audiobookshelf backend mode, improve compatibility with rootless container runtimes, and include cleanup/refactor work and unit tests.

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

@@ -30,6 +30,12 @@ const fsMock = vi.hoisted(() => ({
   access: vi.fn(),
   constants: { R_OK: 4 },
 }));
+const configServiceMock = vi.hoisted(() => ({
+  get: vi.fn(),
+}));
+const downloadClientManagerMock = vi.hoisted(() => ({
+  getAllClients: vi.fn(),
+}));
 
 vi.mock('@/lib/db', () => ({
   prisma: prismaMock,
@@ -73,6 +79,14 @@ vi.mock('fs/promises', () => ({
   ...fsMock,
 }));
 
+vi.mock('@/lib/services/config.service', () => ({
+  getConfigService: () => configServiceMock,
+}));
+
+vi.mock('@/lib/services/download-client-manager.service', () => ({
+  getDownloadClientManager: () => downloadClientManagerMock,
+}));
+
 describe('Admin settings test routes', () => {
   beforeEach(() => {
     vi.clearAllMocks();
@@ -211,7 +225,10 @@ describe('Admin settings test routes', () => {
   });
 
   it('uses stored password when masked password is provided', async () => {
-    prismaMock.configuration.findUnique.mockResolvedValueOnce({ value: 'stored-pass' });
+    // Mock download client manager to return the stored password
+    downloadClientManagerMock.getAllClients.mockResolvedValueOnce([
+      { type: 'qbittorrent', password: 'stored-pass' },
+    ]);
     qbtMock.testConnectionWithCredentials.mockResolvedValueOnce('4.1.0');
     const request = {
       json: vi.fn().mockResolvedValue({
@@ -236,7 +253,8 @@ describe('Admin settings test routes', () => {
   });
 
   it('returns error when masked password is missing in storage', async () => {
-    prismaMock.configuration.findUnique.mockResolvedValueOnce(null);
+    // Mock download client manager to return no matching client
+    downloadClientManagerMock.getAllClients.mockResolvedValueOnce([]);
     const request = {
       json: vi.fn().mockResolvedValue({
         type: 'qbittorrent',