Add series fields to audiobooks and update related logic

Introduces 'series' and 'seriesPart' fields to the Audiobook model and database schema. Updates API routes, file organization, and path template utilities to support series metadata. Enhances chapter merging logic, improves notification backend testing, and expands test coverage for admin and API routes.

vitest.config.ts

@@ -22,6 +22,8 @@ export default defineConfig({
     clearMocks: true,
     mockReset: true,
     restoreMocks: true,
+    testTimeout: 20000,
+    hookTimeout: 20000,
     coverage: {
       provider: 'v8',
       reporter: ['text', 'html', 'lcov'],