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-core.routes.test.ts

@@ -77,6 +77,13 @@ vi.mock('@/lib/services/download-client-manager.service', () => ({
   invalidateDownloadClientManager: invalidateDownloadClientManagerMock,
 }));
 
+vi.mock('@/lib/services/encryption.service', () => ({
+  getEncryptionService: () => ({
+    encrypt: (value: string) => `enc-${value}`,
+    decrypt: (value: string) => value.replace('enc-', ''),
+  }),
+}));
+
 describe('Admin settings core routes', () => {
   beforeEach(() => {
     vi.clearAllMocks();