fix: rTorrent null-safety, configurable SAB_HISTORY_LIMIT, lastError visibility (#68)

- RTorrentClient: guard d.multicall2 returning non-array, per-row try/catch,
  explicit Number()/String() coercions, _extractArrInfo null-safe
- RTorrentClient.getClientStatus: coerce rates through Number.isFinite
- SABnzbdClient: history limit now reads SAB_HISTORY_LIMIT env var (default 10)
- DownloadClient: added _recordLastError, _clearLastError, getLastError on base
- All four clients call _recordLastError on failure, _clearLastError on success
- DownloadClientRegistry.getAllClientStatuses: includes lastError in result
- GET /api/status/status: exposes downloadClients[] array with per-client lastError
- Tests: RTorrentClient null-safety + lastError, SABnzbd history limit + lastError,
  downloadClients.test expectation updated for new lastError field

server/clients/TransmissionClient.js

@@ -18,9 +18,11 @@ class TransmissionClient extends DownloadClient {
     try {
       await this.makeRequest('session-get');
       logToFile(`[Transmission:${this.name}] Connection test successful`);
+      this._clearLastError();
       return true;
     } catch (error) {
       logToFile(`[Transmission:${this.name}] Connection test failed: ${error.message}`);
+      this._recordLastError('testConnection', error);
       return false;
     }
   }
@@ -80,10 +82,11 @@ class TransmissionClient extends DownloadClient {
 
       const torrents = response.data.arguments.torrents || [];
       logToFile(`[Transmission:${this.name}] Retrieved ${torrents.length} torrents`);
-      
+      this._clearLastError();
       return torrents.map(torrent => this.normalizeDownload(torrent));
     } catch (error) {
       logToFile(`[Transmission:${this.name}] Error fetching torrents: ${error.message}`);
+      this._recordLastError('getActiveDownloads', error);
       return [];
     }
   }
@@ -93,12 +96,14 @@ class TransmissionClient extends DownloadClient {
       const response = await this.makeRequest('session-get');
       const sessionStats = await this.makeRequest('session-stats');
       
+      this._clearLastError();
       return {
         session: response.data.arguments,
         stats: sessionStats.data.arguments
       };
     } catch (error) {
       logToFile(`[Transmission:${this.name}] Error getting client status: ${error.message}`);
+      this._recordLastError('getClientStatus', error);
       return null;
     }
   }