Add sourceHeaders and conditional OIDC groups

Add support for passing sourceHeaders when fetching NZB/torrent files: extend AddDownloadOptions and SABnzbd AddNZBOptions, forward headers in sabnzbd and nzbget clients, and populate sourceHeaders in download-torrent.processor (injecting Prowlarr API key as X-Api-Key for proxy URLs). Make OIDC request scope conditional: only include the 'groups' scope when group-based access control or admin-claim is enabled (update provider logic, add tests, and update setup UI text). Also remove explicit take:100 in Plex processors and add CLAUDE guidance about requesting approval before implementing code changes.

src/lib/processors/scan-plex.processor.ts

@@ -450,7 +450,7 @@ export async function processScanPlex(payload: ScanPlexPayload): Promise<any> {
           },
         },
       },
-      take: 100, // Increased from 50 to handle more eligible requests
+
     });
 
     logger.info(`Found ${matchableRequests.length} matchable requests (all non-terminal statuses)`);