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

tests/unit/clients/RTorrentClient.test.js

@@ -420,4 +420,68 @@ describe('RTorrentClient', () => {
       expect(status).toBeNull();
     });
   });
+
+  describe('Null-safety (Issue #68)', () => {
+    it('should return [] when d.multicall2 returns a non-array', async () => {
+      mockMethodCall.mockImplementation((method, params, callback) => {
+        callback(null, null);
+      });
+      const downloads = await client.getActiveDownloads();
+      expect(downloads).toEqual([]);
+    });
+
+    it('should skip malformed individual torrent rows instead of throwing', async () => {
+      const torrents = [
+        // valid row
+        ['hashA', 'Name A', 100, 50, 0, 0, 1, 1, 0, '/dl', ''],
+        // malformed row (not an array)
+        'not-an-array',
+        // row with null/undefined fields
+        ['hashB', null, null, null, null, null, null, null, null, null, null]
+      ];
+      mockMethodCall.mockImplementation((method, params, callback) => {
+        callback(null, torrents);
+      });
+      const downloads = await client.getActiveDownloads();
+      expect(downloads).toHaveLength(2);
+      expect(downloads[0].id).toBe('hashA');
+      expect(downloads[1].id).toBe('hashB');
+      expect(downloads[1].title).toBe('');
+      expect(downloads[1].size).toBe(0);
+    });
+
+    it('_extractArrInfo should return {} for non-string filename', () => {
+      expect(client._extractArrInfo(null)).toEqual({});
+      expect(client._extractArrInfo(undefined)).toEqual({});
+      expect(client._extractArrInfo(123)).toEqual({});
+    });
+  });
+
+  describe('lastError tracking (Issue #68)', () => {
+    it('should record lastError on getActiveDownloads failure', async () => {
+      mockMethodCall.mockImplementation((method, params, callback) => {
+        callback(new Error('boom'));
+      });
+      await client.getActiveDownloads();
+      expect(client.getLastError()).not.toBeNull();
+      expect(client.getLastError().operation).toBe('getActiveDownloads');
+      expect(client.getLastError().message).toBe('boom');
+    });
+
+    it('should clear lastError on successful call', async () => {
+      // First, fail.
+      mockMethodCall.mockImplementationOnce((method, params, callback) => {
+        callback(new Error('boom'));
+      });
+      await client.getActiveDownloads();
+      expect(client.getLastError()).not.toBeNull();
+
+      // Then, succeed.
+      mockMethodCall.mockImplementation((method, params, callback) => {
+        callback(null, []);
+      });
+      await client.getActiveDownloads();
+      expect(client.getLastError()).toBeNull();
+    });
+  });
 });