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/routes/status.js

@@ -7,6 +7,7 @@ const { getLastPollTimings, POLLING_ENABLED } = require('../utils/poller');
 const { getSonarrInstances, getRadarrInstances, getOmbiInstances } = require('../utils/config');
 const { getGlobalWebhookMetrics } = require('../utils/cache');
 const { checkWebhookConfigured, checkOmbiWebhookConfigured, aggregateMetrics } = require('../services/WebhookStatus');
+const downloadClientRegistry = require('../utils/downloadClients');
 
 /**
  * @openapi
@@ -165,7 +166,16 @@ router.get('/', requireAuth, async (req, res) => {
         sonarr: aggregateMetrics(sonarrMetrics, sonarrWebhookConfigured),
         radarr: aggregateMetrics(radarrMetrics, radarrWebhookConfigured),
         ombi: aggregateMetrics(ombiMetrics, ombiWebhookConfigured)
-      }
+      },
+      // Per-download-client health summary including any lastError captured
+      // since the last successful call. Lets the admin status panel surface
+      // transient failures (auth expiry, RPC blips, etc.) without log scraping.
+      downloadClients: downloadClientRegistry.getAllClients().map(c => ({
+        instanceId: c.getInstanceId(),
+        instanceName: c.name,
+        clientType: c.getClientType(),
+        lastError: typeof c.getLastError === 'function' ? c.getLastError() : null
+      }))
     });
   } catch (err) {
     res.status(500).json({ error: 'Failed to get status', details: err.message });