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.

prisma/migrations/20260122000000_add_series_fields/migration.sql

@@ -0,0 +1,3 @@
+-- AddSeriesFields
+ALTER TABLE "audiobooks" ADD COLUMN "series" TEXT;
+ALTER TABLE "audiobooks" ADD COLUMN "series_part" TEXT;

prisma/schema.prisma

@@ -168,6 +168,8 @@ model Audiobook {
   description String? @db.Text
   coverArtUrl String? @map("cover_art_url") @db.Text
   year        Int?    // Release year extracted from releaseDate
+  series      String? // Book series name (e.g., "The Mistborn Saga")
+  seriesPart  String? @map("series_part") // Series position (e.g., "1", "1.5", "Book 1")
 
   // Request tracking
   status String @default("requested") // requested, downloading, processing, completed, failed