Implement Pluggable Download Client Architecture (PDCA)

- Add abstract DownloadClient base class with standardized interface
- Refactor QBittorrentClient to extend DownloadClient with Sync API support
- Create SABnzbdClient implementing DownloadClient interface
- Add TransmissionClient as proof-of-concept implementation
- Implement DownloadClientRegistry for factory pattern and client management
- Refactor poller.js to use unified client interface (30-40% code reduction)
- Maintain 100% backward compatibility with existing cache structure
- Add comprehensive test suite (12 unit + integration tests)
- Update ARCHITECTURE.md with detailed PDCA documentation
- Create ADDING-A-DOWNLOAD-CLIENT.md guide for future client additions

Features:
- Client-agnostic polling with error isolation
- Consistent data normalization across all clients
- Easy extensibility for new download client types
- Zero breaking changes to existing functionality
- Parallel execution with unified timing and logging

server/clients/QBittorrentClient.js

@@ -0,0 +1,256 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+const axios = require('axios');
+const DownloadClient = require('./DownloadClient');
+const { logToFile } = require('../utils/logger');
+
+class QBittorrentClient extends DownloadClient {
+  constructor(instance) {
+    super(instance);
+    this.authCookie = null;
+    // Sync API incremental state
+    this.lastRid = 0;
+    this.torrentMap = new Map();
+    this.fallbackThisCycle = false;
+  }
+
+  getClientType() {
+    return 'qbittorrent';
+  }
+
+  async testConnection() {
+    try {
+      await this.login();
+      // Try a simple API call to verify connection
+      await this.makeRequest('/api/v2/app/version');
+      logToFile(`[qBittorrent:${this.name}] Connection test successful`);
+      return true;
+    } catch (error) {
+      logToFile(`[qBittorrent:${this.name}] Connection test failed: ${error.message}`);
+      return false;
+    }
+  }
+
+  async login() {
+    try {
+      logToFile(`[qBittorrent:${this.name}] Attempting login...`);
+      const response = await axios.post(`${this.url}/api/v2/auth/login`, 
+        `username=${encodeURIComponent(this.username)}&password=${encodeURIComponent(this.password)}`,
+        {
+          headers: {
+            'Content-Type': 'application/x-www-form-urlencoded'
+          },
+          maxRedirects: 0,
+          validateStatus: (status) => status >= 200 && status < 400
+        }
+      );
+      
+      if (response.headers['set-cookie']) {
+        this.authCookie = response.headers['set-cookie'][0];
+        logToFile(`[qBittorrent:${this.name}] Login successful`);
+        return true;
+      }
+      
+      logToFile(`[qBittorrent:${this.name}] Login failed - no cookie`);
+      return false;
+    } catch (error) {
+      logToFile(`[qBittorrent:${this.name}] Login error: ${error.message}`);
+      return false;
+    }
+  }
+
+  async makeRequest(endpoint, config = {}) {
+    const url = `${this.url}${endpoint}`;
+    
+    if (!this.authCookie) {
+      const loggedIn = await this.login();
+      if (!loggedIn) {
+        throw new Error(`Failed to authenticate with ${this.name}`);
+      }
+    }
+    
+    try {
+      const response = await axios.get(url, {
+        ...config,
+        headers: {
+          ...config.headers,
+          'Cookie': this.authCookie
+        }
+      });
+      return response;
+    } catch (error) {
+      // If unauthorized, try re-authenticating once
+      if (error.response && error.response.status === 403) {
+        logToFile(`[qBittorrent:${this.name}] Auth expired, re-authenticating...`);
+        this.authCookie = null;
+        const loggedIn = await this.login();
+        if (loggedIn) {
+          return axios.get(url, {
+            ...config,
+            headers: {
+              ...config.headers,
+              'Cookie': this.authCookie
+            }
+          });
+        }
+      }
+      throw error;
+    }
+  }
+
+  /**
+   * Fetches incremental torrent data using the qBittorrent Sync API.
+   */
+  async getMainData() {
+    const response = await this.makeRequest(`/api/v2/sync/maindata?rid=${this.lastRid}`);
+    const data = response.data;
+
+    if (data.full_update) {
+      // Full refresh: rebuild the entire map
+      this.torrentMap.clear();
+      if (data.torrents) {
+        for (const [hash, props] of Object.entries(data.torrents)) {
+          this.torrentMap.set(hash, { ...props, hash });
+        }
+      }
+    } else {
+      // Delta update: merge changed fields into existing torrent objects
+      if (data.torrents) {
+        for (const [hash, delta] of Object.entries(data.torrents)) {
+          const existing = this.torrentMap.get(hash) || { hash };
+          this.torrentMap.set(hash, { ...existing, ...delta });
+        }
+      }
+    }
+
+    // Remove torrents that the server reports as deleted
+    if (data.torrents_removed) {
+      for (const hash of data.torrents_removed) {
+        this.torrentMap.delete(hash);
+      }
+    }
+
+    // Ensure every torrent has a computed 'completed' field for downstream consumers
+    for (const torrent of this.torrentMap.values()) {
+      if (torrent.completed === undefined && torrent.size !== undefined && torrent.progress !== undefined) {
+        torrent.completed = Math.round(torrent.size * torrent.progress);
+      }
+    }
+
+    this.lastRid = data.rid;
+    return Array.from(this.torrentMap.values());
+  }
+
+  /**
+   * Legacy full-list fetch. Used as a fallback when the Sync API fails.
+   */
+  async getTorrentsLegacy() {
+    try {
+      const response = await this.makeRequest('/api/v2/torrents/info');
+      logToFile(`[qBittorrent:${this.name}] Retrieved ${response.data.length} torrents (legacy)`);
+      return response.data;
+    } catch (error) {
+      logToFile(`[qBittorrent:${this.name}] Error fetching torrents (legacy): ${error.message}`);
+      throw error;
+    }
+  }
+
+  async getActiveDownloads() {
+    try {
+      if (this.fallbackThisCycle) {
+        logToFile(`[qBittorrent:${this.name}] Already fell back this cycle, using legacy`);
+        const torrents = await this.getTorrentsLegacy();
+        return torrents.map(torrent => this.normalizeDownload(torrent));
+      }
+
+      const torrents = await this.getMainData();
+      logToFile(`[qBittorrent:${this.name}] Sync: ${torrents.length} torrents (rid=${this.lastRid})`);
+      return torrents.map(torrent => this.normalizeDownload(torrent));
+    } catch (error) {
+      logToFile(`[qBittorrent:${this.name}] Sync failed, falling back to legacy: ${error.message}`);
+      this.fallbackThisCycle = true;
+      try {
+        const torrents = await this.getTorrentsLegacy();
+        return torrents.map(torrent => this.normalizeDownload(torrent));
+      } catch (fallbackError) {
+        logToFile(`[qBittorrent:${this.name}] Fallback also failed: ${fallbackError.message}`);
+        return [];
+      }
+    }
+  }
+
+  async getClientStatus() {
+    try {
+      const response = await this.makeRequest('/api/v2/sync/maindata');
+      const data = response.data;
+      
+      return {
+        serverState: data.server_state || {},
+        rid: data.rid,
+        fullUpdate: data.full_update
+      };
+    } catch (error) {
+      logToFile(`[qBittorrent:${this.name}] Error getting client status: ${error.message}`);
+      return null;
+    }
+  }
+
+  normalizeDownload(torrent) {
+    const totalSize = torrent.size;
+    const downloadedSize = torrent.completed || Math.round(torrent.size * torrent.progress);
+    const progress = torrent.progress * 100;
+    
+    // Map qBittorrent states to our normalized status
+    const stateMap = {
+      'downloading': 'Downloading',
+      'stalledDL': 'Downloading',
+      'metaDL': 'Downloading',
+      'forcedDL': 'Downloading',
+      'allocating': 'Downloading',
+      'uploading': 'Seeding',
+      'stalledUP': 'Seeding',
+      'forcedUP': 'Seeding',
+      'queuedUP': 'Queued',
+      'queuedDL': 'Queued',
+      'checkingUP': 'Checking',
+      'checkingDL': 'Checking',
+      'checkingResumeData': 'Checking',
+      'moving': 'Moving',
+      'pausedUP': 'Paused',
+      'pausedDL': 'Paused',
+      'stoppedUP': 'Stopped',
+      'stoppedDL': 'Stopped',
+      'error': 'Error',
+      'missingFiles': 'Error',
+      'unknown': 'Unknown'
+    };
+    
+    const status = stateMap[torrent.state] || torrent.state;
+    
+    return {
+      id: torrent.hash,
+      title: torrent.name,
+      type: 'torrent',
+      client: 'qbittorrent',
+      instanceId: this.id,
+      instanceName: this.name,
+      status: status,
+      progress: Math.round(progress),
+      size: totalSize,
+      downloaded: downloadedSize,
+      speed: torrent.dlspeed,
+      eta: torrent.eta < 0 || torrent.eta === 8640000 ? null : torrent.eta,
+      category: torrent.category || undefined,
+      tags: torrent.tags ? torrent.tags.split(',').filter(tag => tag.trim()) : [],
+      savePath: torrent.content_path || torrent.save_path || undefined,
+      addedOn: torrent.added_on ? new Date(torrent.added_on * 1000).toISOString() : undefined,
+      raw: torrent
+    };
+  }
+
+  // Reset fallback flag (called by registry at start of each poll cycle)
+  resetFallbackFlag() {
+    this.fallbackThisCycle = false;
+  }
+}
+
+module.exports = QBittorrentClient;