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/SABnzbdClient.js

@@ -3,9 +3,27 @@ const axios = require('axios');
 const DownloadClient = require('./DownloadClient');
 const { logToFile } = require('../utils/logger');
 
+// Number of recently completed jobs to pull from SABnzbd's /api?mode=history on
+// every poll. Larger values let DownloadMatcher correlate slightly older jobs
+// with their Sonarr/Radarr queue entries at the cost of one wider HTTP
+// response per poll cycle. Configurable via the SAB_HISTORY_LIMIT environment
+// variable; defaults to 10 to match the previous hardcoded value.
+const DEFAULT_HISTORY_LIMIT = 10;
+function resolveHistoryLimit() {
+  const raw = process.env.SAB_HISTORY_LIMIT;
+  if (raw === undefined || raw === null || raw === '') return DEFAULT_HISTORY_LIMIT;
+  const parsed = parseInt(raw, 10);
+  if (!Number.isFinite(parsed) || parsed < 0) {
+    logToFile(`[SABnzbd] Invalid SAB_HISTORY_LIMIT='${raw}'; falling back to ${DEFAULT_HISTORY_LIMIT}`);
+    return DEFAULT_HISTORY_LIMIT;
+  }
+  return parsed;
+}
+
 class SABnzbdClient extends DownloadClient {
   constructor(instance) {
     super(instance);
+    this.historyLimit = resolveHistoryLimit();
   }
 
   getClientType() {
@@ -16,9 +34,11 @@ class SABnzbdClient extends DownloadClient {
     try {
       const response = await this.makeRequest('', { mode: 'version' });
       logToFile(`[SABnzbd:${this.name}] Connection test successful, version: ${response.data.version}`);
+      this._clearLastError();
       return true;
     } catch (error) {
       logToFile(`[SABnzbd:${this.name}] Connection test failed: ${error.message}`);
+      this._recordLastError('testConnection', error);
       return false;
     }
   }
@@ -47,7 +67,7 @@ class SABnzbdClient extends DownloadClient {
       // Get both queue and history to provide complete picture
       const [queueResponse, historyResponse] = await Promise.all([
         this.makeRequest({ mode: 'queue' }),
-        this.makeRequest({ mode: 'history', limit: 10 })
+        this.makeRequest({ mode: 'history', limit: this.historyLimit })
       ]);
 
       const queueData = queueResponse.data;
@@ -99,10 +119,12 @@ class SABnzbdClient extends DownloadClient {
         }
       }
 
-      logToFile(`[SABnzbd:${this.name}] Retrieved ${downloads.length} downloads`);
+      logToFile(`[SABnzbd:${this.name}] Retrieved ${downloads.length} downloads (historyLimit=${this.historyLimit})`);
+      this._clearLastError();
       return downloads;
     } catch (error) {
       logToFile(`[SABnzbd:${this.name}] Error fetching downloads: ${error.message}`);
+      this._recordLastError('getActiveDownloads', error);
       return [];
     }
   }
@@ -112,8 +134,12 @@ class SABnzbdClient extends DownloadClient {
       const response = await this.makeRequest({ mode: 'queue' });
       const queueData = response.data.queue;
       
-      if (!queueData) return null;
+      if (!queueData) {
+        this._clearLastError();
+        return null;
+      }
 
+      this._clearLastError();
       return {
         status: queueData.status,
         speed: queueData.speed,
@@ -128,6 +154,7 @@ class SABnzbdClient extends DownloadClient {
       };
     } catch (error) {
       logToFile(`[SABnzbd:${this.name}] Error getting client status: ${error.message}`);
+      this._recordLastError('getClientStatus', error);
       return null;
     }
   }