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.

src/lib/utils/path-template.util.ts

@@ -15,6 +15,8 @@ export interface TemplateVariables {
   narrator?: string;
   asin?: string;
   year?: number;
+  series?: string;
+  seriesPart?: string;
 }
 
 /**
@@ -28,7 +30,7 @@ export interface ValidationResult {
 /**
  * Supported template variable names
  */
-const VALID_VARIABLES = ['author', 'title', 'narrator', 'asin', 'year'];
+const VALID_VARIABLES = ['author', 'title', 'narrator', 'asin', 'year', 'series', 'seriesPart'];
 
 /**
  * Invalid file path characters (outside of template variables)
@@ -228,14 +230,18 @@ export function generateMockPreviews(template: string): string[] {
       title: 'Mistborn: The Final Empire',
       narrator: 'Michael Kramer',
       asin: 'B002UZMLXM',
-      year: 2006
+      year: 2006,
+      series: 'The Mistborn Saga',
+      seriesPart: '1'
     },
     {
       author: 'Douglas Adams',
       title: "The Hitchhiker's Guide to the Galaxy",
       narrator: 'Stephen Fry',
       asin: 'B0009JKV9W',
-      year: 2005
+      year: 2005,
+      series: "Hitchhiker's Guide",
+      seriesPart: '1'
     },
     {
       author: 'Andy Weir',
@@ -243,6 +249,7 @@ export function generateMockPreviews(template: string): string[] {
       // No narrator for this example
       asin: 'B08G9PRS1K',
       year: 2021
+      // No series data - to test empty handling
     }
   ];