Add SABnzbd Usenet/NZB integration and documentation

Introduces SABnzbd as a supported download client for Usenet/NZB alongside qBittorrent, including service implementation, setup wizard and admin settings UI updates, and protocol-specific job processor logic. Updates documentation, PRD, and database schema to support NZB downloads, adds comprehensive technical details and testing strategies, and fixes Audible integration issues related to search and ASIN extraction.

src/lib/services/job-queue.service.ts

@@ -51,7 +51,7 @@ export interface MonitorDownloadPayload extends JobPayload {
   requestId: string;
   downloadHistoryId: string;
   downloadClientId: string;
-  downloadClient: 'qbittorrent' | 'transmission';
+  downloadClient: 'qbittorrent' | 'sabnzbd';
 }
 
 export interface OrganizeFilesPayload extends JobPayload {
@@ -479,7 +479,7 @@ export class JobQueueService {
     requestId: string,
     downloadHistoryId: string,
     downloadClientId: string,
-    downloadClient: 'qbittorrent' | 'transmission',
+    downloadClient: 'qbittorrent' | 'sabnzbd',
     delaySeconds: number = 0
   ): Promise<string> {
     return await this.addJob(