SABnzbd path mapping + ASIN-based request deletion

Add bidirectional path mapping and complete_dir-aware category sync to the SABnzbd integration. Introduces PathMapper usage, complete_dir extraction, calculateCategoryPath(), and ensureCategory() logic to choose empty/relative/absolute category paths; ensureCategory is invoked before adding NZBs. Update singleton factory to load download_dir and path-mapping config from DownloadClientManager and recreate the service when config is not loaded. Make DownloadClientManager pass path-mapping config into the SABnzbd service. Change request deletion to remove plex_library records by ASIN (deleteMany) with a fallback to exact title/author matches so availability checks and deletions are consistent. Update documentation and tests to reflect the new behavior and APIs.

src/lib/services/download-client-manager.service.ts

@@ -196,11 +196,21 @@ export class DownloadClientManager {
    * Create SABnzbd service instance
    */
   private createSABnzbdService(config: DownloadClientConfig): SABnzbdService {
+    const pathMapping: PathMappingConfig | undefined = config.remotePathMappingEnabled && config.remotePath && config.localPath
+      ? {
+          enabled: true,
+          remotePath: config.remotePath,
+          localPath: config.localPath,
+        }
+      : undefined;
+
     return new SABnzbdService(
       config.url,
       config.password, // API key stored in password field
       config.category || 'readmeabook', // defaultCategory
-      config.disableSSLVerify
+      '/downloads', // defaultDownloadDir (will be overridden by singleton with actual config)
+      config.disableSSLVerify,
+      pathMapping
     );
   }