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/DownloadClient.js

@@ -25,6 +25,41 @@ class DownloadClient {
     this.apiKey = instanceConfig.apiKey;
     this.username = instanceConfig.username;
     this.password = instanceConfig.password;
+
+    // Last error encountered while talking to this client.
+    // Cleared on successful calls via _clearLastError(); set via _recordLastError().
+    // Surfaced through getAllClientStatuses() so the admin status panel can show
+    // a per-client failure indicator without needing to scrape logs.
+    this.lastError = null;
+  }
+
+  /**
+   * Record an error encountered while talking to this client.
+   * @param {string} operation - Short description of the operation (e.g. 'getActiveDownloads')
+   * @param {Error|string} error - Error object or message
+   */
+  _recordLastError(operation, error) {
+    const message = (error && error.message) ? error.message : String(error || 'unknown error');
+    this.lastError = {
+      operation,
+      message,
+      at: new Date().toISOString()
+    };
+  }
+
+  /**
+   * Clear the last error (called when an operation succeeds).
+   */
+  _clearLastError() {
+    this.lastError = null;
+  }
+
+  /**
+   * Public accessor for the last recorded error, or null if none.
+   * @returns {{operation:string, message:string, at:string}|null}
+   */
+  getLastError() {
+    return this.lastError;
   }
 
   /**