Add User-Agent header to Prowlarr RSS queries

Set User-Agent to "ReadMeABook" on the Newznab proxy RSS endpoint
so RMAB is identifiable in Prowlarr stats instead of showing as
generic "axios". Sonarr/Radarr already do this with their own
User-Agent strings.

Only applies to the RSS feed endpoint (/{indexerId}/api) which
respects User-Agent for Source identification. The /api/v1/search
endpoint hardcodes Source as "Prowlarr" regardless of headers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/integrations/prowlarr.service.ts

@@ -315,6 +315,9 @@ export class ProwlarrService {
           limit: 100,
           extended: 1,
         },
+        headers: {
+          'User-Agent': 'ReadMeABook',
+        },
         timeout: DOWNLOAD_CLIENT_TIMEOUT,
         responseType: 'text', // Get XML as text
       });