fix(downloads): Fix SABnzbd/qBittorrent collision and webhook metrics

1. Fixed download client collision:
   - SABnzbd client with id 'i3omb' was being overwritten by qBittorrent
   - Now uses unique key ':' like the arr retrievers

2. Fixed webhook metrics showing 0:
   - instanceName from webhooks is generic ('Sonarr', 'Radarr')
   - Not the configured instance name ('i3omb')
   - Now updates metrics for ALL instances of that type

server/utils/downloadClients.js

@@ -63,8 +63,9 @@ class DownloadClientRegistry {
         }
 
         const client = new ClientClass(config);
-        this.clients.set(config.id, client);
-        logToFile(`[DownloadClientRegistry] Created ${config.type} client: ${config.name} (${config.id})`);
+        const uniqueKey = `${config.type}:${config.id}`;
+        this.clients.set(uniqueKey, client);
+        logToFile(`[DownloadClientRegistry] Created ${config.type} client: ${config.name} (${uniqueKey})`);
       } catch (error) {
         logToFile(`[DownloadClientRegistry] Failed to create client ${config.id}: ${error.message}`);
       }