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/utils/downloadClients.js

@@ -239,7 +239,10 @@ class DownloadClientRegistry {
             instanceId: client.getInstanceId(),
             instanceName: client.name,
             clientType: client.getClientType(),
-            status
+            status,
+            // Surface the per-client lastError so admins can see transient
+            // failures (auth expiry, RPC blips, etc.) without scraping logs.
+            lastError: typeof client.getLastError === 'function' ? client.getLastError() : null
           };
         } catch (error) {
           logToFile(`[DownloadClientRegistry] Error getting status from ${client.name}: ${error.message}`);
@@ -248,7 +251,8 @@ class DownloadClientRegistry {
             instanceName: client.name,
             clientType: client.getClientType(),
             status: null,
-            error: error.message
+            error: error.message,
+            lastError: typeof client.getLastError === 'function' ? client.getLastError() : null
           };
         }
       })