refactor: use qBittorrent Sync API (/api/v2/sync/maindata) with fallback

- QBittorrentClient now uses the incremental Sync API instead of repeatedly
  fetching the full torrent list via /api/v2/torrents/info.
- Per-client state: lastRid, torrentMap, fallbackThisCycle.
- Handles full_update, delta updates, and torrents_removed.
- Falls back to legacy torrents/info at most once per poll cycle.
- getAllTorrents() resets fallback flags before each cycle.
- Added 9 new unit tests covering: first sync, delta merge, full_update,
  torrents_removed, fallback path, direct-legacy-after-fallback, 403 re-auth,
  completed-field computation, and fallback reset.

tests/unit/qbittorrent.test.js

@@ -7,7 +7,8 @@
  * dashboard card rendering so correctness matters for UX.
  */
 
-import { mapTorrentToDownload, formatBytes, formatSpeed, formatEta } from '../../server/utils/qbittorrent.js';
+import { mapTorrentToDownload, formatBytes, formatSpeed, formatEta, QBittorrentClient } from '../../server/utils/qbittorrent.js';
+import nock from 'nock';
 
 // Minimal torrent fixture that satisfies mapTorrentToDownload's expectations
 function makeTorrent(overrides = {}) {
@@ -32,6 +33,35 @@ function makeTorrent(overrides = {}) {
   };
 }
 
+const QBT_URL = 'http://qbittorrent.test:8080';
+
+function makeClient(overrides = {}) {
+  return new QBittorrentClient({
+    id: 'test-qbt',
+    name: 'TestQBT',
+    url: QBT_URL,
+    username: 'admin',
+    password: 'adminadmin',
+    ...overrides
+  });
+}
+
+function mockLogin() {
+  return nock(QBT_URL)
+    .post('/api/v2/auth/login')
+    .reply(200, {}, { 'set-cookie': ['SID=abc123; path=/'] });
+}
+
+function mockSync(rid, response) {
+  return nock(QBT_URL)
+    .get(`/api/v2/sync/maindata?rid=${rid}`)
+    .reply(200, response);
+}
+
+afterEach(() => {
+  nock.cleanAll();
+});
+
 describe('formatBytes', () => {
   it('formats 0 bytes', () => expect(formatBytes(0)).toBe('0 B'));
   it('formats bytes', () => expect(formatBytes(512)).toBe('512 B'));
@@ -110,3 +140,230 @@ describe('mapTorrentToDownload', () => {
     expect(result.savePath).toBe('/dl/');
   });
 });
+
+describe('QBittorrentClient sync API', () => {
+  it('first call uses rid=0 and returns full torrent list', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+
+    mockSync(0, {
+      rid: 1,
+      full_update: true,
+      torrents: {
+        hash01: { name: 'Test1', state: 'downloading', size: 1000, progress: 0.5, dlspeed: 100, eta: 60, num_seeds: 5, num_leechs: 2, availability: 1.0 }
+      }
+    });
+
+    const torrents = await client.getTorrents();
+    expect(torrents).toHaveLength(1);
+    expect(torrents[0].name).toBe('Test1');
+    expect(torrents[0].instanceId).toBe('test-qbt');
+    expect(torrents[0].hash).toBe('hash01');
+    expect(client.lastRid).toBe(1);
+  });
+
+  it('subsequent call uses last rid and merges delta', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+
+    // First call
+    mockSync(0, {
+      rid: 1,
+      full_update: true,
+      torrents: {
+        hash01: { name: 'Test1', state: 'downloading', size: 1000, progress: 0.5, dlspeed: 100, eta: 60, num_seeds: 5, num_leechs: 2, availability: 1.0 }
+      }
+    });
+    await client.getTorrents();
+
+    // Second call — delta
+    mockSync(1, {
+      rid: 2,
+      full_update: false,
+      torrents: {
+        hash01: { dlspeed: 200 }
+      }
+    });
+
+    const torrents = await client.getTorrents();
+    expect(torrents).toHaveLength(1);
+    expect(torrents[0].dlspeed).toBe(200);
+    expect(torrents[0].name).toBe('Test1');
+    expect(client.lastRid).toBe(2);
+  });
+
+  it('handles full_update=true on subsequent call', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+
+    // First call
+    mockSync(0, {
+      rid: 1,
+      full_update: true,
+      torrents: {
+        hash01: { name: 'Test1', state: 'downloading', size: 1000, progress: 0.5, dlspeed: 100, eta: 60, num_seeds: 5, num_leechs: 2, availability: 1.0 }
+      }
+    });
+    await client.getTorrents();
+
+    // Server forces full refresh
+    mockSync(1, {
+      rid: 2,
+      full_update: true,
+      torrents: {
+        hash02: { name: 'Test2', state: 'uploading', size: 2000, progress: 1.0, dlspeed: 0, eta: 0, num_seeds: 10, num_leechs: 0, availability: 1.0 }
+      }
+    });
+
+    const torrents = await client.getTorrents();
+    expect(torrents).toHaveLength(1);
+    expect(torrents[0].name).toBe('Test2');
+    expect(torrents[0].hash).toBe('hash02');
+    expect(client.lastRid).toBe(2);
+  });
+
+  it('removes torrents when torrents_removed is present', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+
+    mockSync(0, {
+      rid: 1,
+      full_update: true,
+      torrents: {
+        hash01: { name: 'Test1', state: 'downloading', size: 1000, progress: 0.5, dlspeed: 100, eta: 60, num_seeds: 5, num_leechs: 2, availability: 1.0 }
+      }
+    });
+    await client.getTorrents();
+
+    mockSync(1, {
+      rid: 2,
+      full_update: false,
+      torrents_removed: ['hash01']
+    });
+
+    const torrents = await client.getTorrents();
+    expect(torrents).toHaveLength(0);
+  });
+
+  it('falls back to torrents/info when sync fails', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+
+    // Sync fails with 500
+    nock(QBT_URL)
+      .get('/api/v2/sync/maindata?rid=0')
+      .reply(500, { error: 'Internal Server Error' });
+
+    // Legacy succeeds
+    nock(QBT_URL)
+      .get('/api/v2/torrents/info')
+      .reply(200, [
+        { name: 'Fallback', hash: 'fb01', state: 'downloading', size: 1073741824, progress: 0.5, dlspeed: 1048576, eta: 512, num_seeds: 10, num_leechs: 3, availability: 1.0 }
+      ]);
+
+    const torrents = await client.getTorrents();
+    expect(torrents).toHaveLength(1);
+    expect(torrents[0].name).toBe('Fallback');
+    expect(client.fallbackThisCycle).toBe(true);
+  });
+
+  it('uses legacy immediately if already fell back this cycle', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+
+    client.fallbackThisCycle = true;
+
+    // Only legacy should be called
+    nock(QBT_URL)
+      .get('/api/v2/torrents/info')
+      .reply(200, [
+        { name: 'DirectLegacy', hash: 'dl01', state: 'downloading', size: 1000, progress: 0.5, dlspeed: 100, eta: 60, num_seeds: 5, num_leechs: 2, availability: 1.0 }
+      ]);
+
+    // Ensure sync is NOT called
+    const syncScope = nock(QBT_URL)
+      .get('/api/v2/sync/maindata?rid=0')
+      .reply(200, { rid: 1, full_update: true });
+
+    const torrents = await client.getTorrents();
+    expect(torrents).toHaveLength(1);
+    expect(torrents[0].name).toBe('DirectLegacy');
+    expect(syncScope.isDone()).toBe(false);
+  });
+
+  it('re-authenticates on 403 during sync and retries', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+
+    // First sync call returns 403
+    nock(QBT_URL)
+      .get('/api/v2/sync/maindata?rid=0')
+      .reply(403, {});
+
+    // Re-login
+    nock(QBT_URL)
+      .post('/api/v2/auth/login')
+      .reply(200, {}, { 'set-cookie': ['SID=newtoken; path=/'] });
+
+    // Retry succeeds
+    nock(QBT_URL)
+      .get('/api/v2/sync/maindata?rid=0')
+      .reply(200, {
+        rid: 1,
+        full_update: true,
+        torrents: {
+          hash01: { name: 'AfterReauth', state: 'downloading', size: 1000, progress: 0.5, dlspeed: 100, eta: 60, num_seeds: 5, num_leechs: 2, availability: 1.0 }
+        }
+      });
+
+    const torrents = await client.getTorrents();
+    expect(torrents).toHaveLength(1);
+    expect(torrents[0].name).toBe('AfterReauth');
+  });
+
+  it('computes completed from size and progress when missing', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+
+    mockSync(0, {
+      rid: 1,
+      full_update: true,
+      torrents: {
+        hash01: { name: 'NoCompleted', state: 'downloading', size: 1000, progress: 0.5, dlspeed: 100, eta: 60, num_seeds: 5, num_leechs: 2, availability: 1.0 }
+      }
+    });
+
+    const torrents = await client.getTorrents();
+    expect(torrents[0].completed).toBe(500);
+  });
+
+  it('resets fallback flag when getAllTorrents resets it', async () => {
+    mockLogin();
+    const client = makeClient();
+    await client.login();
+    client.fallbackThisCycle = true;
+
+    // After reset, sync should be attempted
+    mockSync(0, {
+      rid: 1,
+      full_update: true,
+      torrents: {
+        hash01: { name: 'ResetWorks', state: 'downloading', size: 1000, progress: 0.5, dlspeed: 100, eta: 60, num_seeds: 5, num_leechs: 2, availability: 1.0 }
+      }
+    });
+
+    // Simulate the reset that getAllTorrents performs
+    client.fallbackThisCycle = false;
+    const torrents = await client.getTorrents();
+    expect(torrents[0].name).toBe('ResetWorks');
+    expect(client.fallbackThisCycle).toBe(false);
+  });
+});