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/QBittorrentClient.js

@@ -23,9 +23,11 @@ class QBittorrentClient extends DownloadClient {
       // Try a simple API call to verify connection
       await this.makeRequest('/api/v2/app/version');
       logToFile(`[qBittorrent:${this.name}] Connection test successful`);
+      this._clearLastError();
       return true;
     } catch (error) {
       logToFile(`[qBittorrent:${this.name}] Connection test failed: ${error.message}`);
+      this._recordLastError('testConnection', error);
       return false;
     }
   }
@@ -174,6 +176,7 @@ class QBittorrentClient extends DownloadClient {
 
       const torrents = await this.getMainData();
       logToFile(`[qBittorrent:${this.name}] Sync: ${torrents.length} torrents (rid=${this.lastRid})`);
+      this._clearLastError();
       return torrents.map(torrent => this.normalizeDownload(torrent));
     } catch (error) {
       logToFile(`[qBittorrent:${this.name}] Sync failed, falling back to legacy: ${error.message}`);
@@ -188,6 +191,7 @@ class QBittorrentClient extends DownloadClient {
         return torrents.map(torrent => this.normalizeDownload(torrent));
       } catch (fallbackError) {
         logToFile(`[qBittorrent:${this.name}] Fallback also failed: ${fallbackError.message}`);
+        this._recordLastError('getActiveDownloads', fallbackError);
         return [];
       }
     }
@@ -198,6 +202,7 @@ class QBittorrentClient extends DownloadClient {
       const response = await this.makeRequest('/api/v2/sync/maindata');
       const data = response.data;
       
+      this._clearLastError();
       return {
         serverState: data.server_state || {},
         rid: data.rid,
@@ -205,6 +210,7 @@ class QBittorrentClient extends DownloadClient {
       };
     } catch (error) {
       logToFile(`[qBittorrent:${this.name}] Error getting client status: ${error.message}`);
+      this._recordLastError('getClientStatus', error);
       return null;
     }
   }