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/SABnzbdClient.js

@@ -0,0 +1,239 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+const axios = require('axios');
+const DownloadClient = require('./DownloadClient');
+const { logToFile } = require('../utils/logger');
+
+class SABnzbdClient extends DownloadClient {
+  constructor(instance) {
+    super(instance);
+  }
+
+  getClientType() {
+    return 'sabnzbd';
+  }
+
+  async testConnection() {
+    try {
+      const response = await this.makeRequest('', { mode: 'version' });
+      logToFile(`[SABnzbd:${this.name}] Connection test successful, version: ${response.data.version}`);
+      return true;
+    } catch (error) {
+      logToFile(`[SABnzbd:${this.name}] Connection test failed: ${error.message}`);
+      return false;
+    }
+  }
+
+  async makeRequest(additionalParams = {}, config = {}) {
+    const params = {
+      output: 'json',
+      apikey: this.apiKey,
+      ...additionalParams
+    };
+
+    try {
+      const response = await axios.get(`${this.url}/api`, {
+        params,
+        ...config
+      });
+      return response;
+    } catch (error) {
+      logToFile(`[SABnzbd:${this.name}] API request failed: ${error.message}`);
+      throw error;
+    }
+  }
+
+  async getActiveDownloads() {
+    try {
+      // Get both queue and history to provide complete picture
+      const [queueResponse, historyResponse] = await Promise.all([
+        this.makeRequest({ mode: 'queue' }),
+        this.makeRequest({ mode: 'history', limit: 10 })
+      ]);
+
+      const queueData = queueResponse.data;
+      const historyData = historyResponse.data;
+
+      const downloads = [];
+
+      // Process active queue items
+      if (queueData.queue && queueData.queue.slots) {
+        for (const slot of queueData.queue.slots) {
+          downloads.push(this.normalizeDownload(slot, 'queue'));
+        }
+      }
+
+      // Process recent history items (last 10)
+      if (historyData.history && historyData.history.slots) {
+        for (const slot of historyData.history.slots) {
+          downloads.push(this.normalizeDownload(slot, 'history'));
+        }
+      }
+
+      logToFile(`[SABnzbd:${this.name}] Retrieved ${downloads.length} downloads`);
+      return downloads;
+    } catch (error) {
+      logToFile(`[SABnzbd:${this.name}] Error fetching downloads: ${error.message}`);
+      return [];
+    }
+  }
+
+  async getClientStatus() {
+    try {
+      const response = await this.makeRequest({ mode: 'queue' });
+      const queueData = response.data.queue;
+      
+      if (!queueData) return null;
+
+      return {
+        status: queueData.status,
+        speed: queueData.speed,
+        kbpersec: queueData.kbpersec,
+        sizeleft: queueData.sizeleft,
+        mbleft: queueData.mbleft,
+        mb: queueData.mb,
+        diskspace1: queueData.diskspace1,
+        diskspace2: queueData.diskspace2,
+        loadavg: queueData.loadavg,
+        pause_int: queueData.pause_int
+      };
+    } catch (error) {
+      logToFile(`[SABnzbd:${this.name}] Error getting client status: ${error.message}`);
+      return null;
+    }
+  }
+
+  normalizeDownload(slot, source) {
+    const isHistory = source === 'history';
+    
+    // Map SABnzbd statuses to normalized status
+    const statusMap = {
+      'Downloading': 'Downloading',
+      'Paused': 'Paused',
+      'Waiting': 'Queued',
+      'Completed': 'Completed',
+      'Failed': 'Error',
+      'Verifying': 'Checking',
+      'Extracting': 'Extracting',
+      'Moving': 'Moving',
+      'QuickCheck': 'Checking',
+      'Repairing': 'Repairing'
+    };
+
+    const status = statusMap[slot.status] || slot.status;
+
+    // Calculate progress
+    let progress = 0;
+    let downloaded = 0;
+    let size = 0;
+
+    if (slot.mb && slot.mbleft !== undefined) {
+      size = slot.mb * 1024 * 1024; // Convert MB to bytes
+      downloaded = (slot.mb - slot.mbleft) * 1024 * 1024;
+      progress = slot.mb > 0 ? ((slot.mb - slot.mbleft) / slot.mb) * 100 : 0;
+    } else if (slot.size) {
+      // Try to parse size string (e.g., "1.5 GB")
+      const sizeMatch = slot.size.match(/^([\d.]+)\s*(\w+)$/i);
+      if (sizeMatch) {
+        const [, sizeValue, sizeUnit] = sizeMatch;
+        const multiplier = this.getUnitMultiplier(sizeUnit);
+        size = parseFloat(sizeValue) * multiplier;
+        
+        if (slot.sizeleft) {
+          const leftMatch = slot.sizeleft.match(/^([\d.]+)\s*(\w+)$/i);
+          if (leftMatch) {
+            const [, leftValue, leftUnit] = leftMatch;
+            const leftMultiplier = this.getUnitMultiplier(leftUnit);
+            downloaded = size - (parseFloat(leftValue) * leftMultiplier);
+            progress = size > 0 ? (downloaded / size) * 100 : 0;
+          }
+        }
+      }
+    }
+
+    // Extract Sonarr/Radarr info from nzb_name if present
+    const arrInfo = this.extractArrInfo(slot.nzb_name || slot.filename || '');
+
+    return {
+      id: slot.nzo_id || slot.id,
+      title: slot.filename || slot.nzb_name || 'Unknown',
+      type: 'usenet',
+      client: 'sabnzbd',
+      instanceId: this.id,
+      instanceName: this.name,
+      status: status,
+      progress: Math.round(progress),
+      size: Math.round(size),
+      downloaded: Math.round(downloaded),
+      speed: slot.kbpersec ? slot.kbpersec * 1024 : 0, // Convert KB/s to bytes/s
+      eta: this.calculateEta(slot.timeleft || slot.eta),
+      category: slot.cat || undefined,
+      tags: slot.labels ? slot.labels.split(',').filter(tag => tag.trim()) : [],
+      savePath: slot.final_name || undefined,
+      addedOn: slot.added ? new Date(slot.added * 1000).toISOString() : undefined,
+      arrQueueId: arrInfo.queueId,
+      arrType: arrInfo.type,
+      raw: { ...slot, source }
+    };
+  }
+
+  getUnitMultiplier(unit) {
+    const unitMap = {
+      'b': 1,
+      'byte': 1,
+      'bytes': 1,
+      'kb': 1024,
+      'k': 1024,
+      'mb': 1024 * 1024,
+      'm': 1024 * 1024,
+      'gb': 1024 * 1024 * 1024,
+      'g': 1024 * 1024 * 1024,
+      'tb': 1024 * 1024 * 1024 * 1024,
+      't': 1024 * 1024 * 1024 * 1024
+    };
+    return unitMap[unit.toLowerCase()] || 1;
+  }
+
+  calculateEta(timeLeft) {
+    if (!timeLeft || timeLeft === '0:00' || timeLeft === 'unknown') {
+      return null;
+    }
+
+    // Parse time in various formats: "0:05:30", "15:30", "330"
+    const parts = timeLeft.split(':').reverse();
+    let totalSeconds = 0;
+
+    if (parts.length === 1) {
+      // Just seconds
+      totalSeconds = parseInt(parts[0], 10);
+    } else if (parts.length === 2) {
+      // MM:SS
+      totalSeconds = parseInt(parts[0], 10) + parseInt(parts[1], 10) * 60;
+    } else if (parts.length === 3) {
+      // HH:MM:SS
+      totalSeconds = parseInt(parts[0], 10) + parseInt(parts[1], 10) * 60 + parseInt(parts[2], 10) * 3600;
+    }
+
+    return isNaN(totalSeconds) ? null : totalSeconds;
+  }
+
+  extractArrInfo(filename) {
+    // Try to extract Sonarr/Radarr info from filename patterns
+    // This is a simple implementation - could be enhanced with regex patterns
+    
+    // Look for patterns like "Series Name - S01E02 - Episode Title"
+    const seriesMatch = filename.match(/[-\s]S(\d{2})E(\d{2})/i);
+    if (seriesMatch) {
+      return { type: 'series' };
+    }
+
+    // Look for movie year patterns like "Movie Title (2023)"
+    const movieMatch = filename.match(/\((\d{4})\)/);
+    if (movieMatch && !seriesMatch) {
+      return { type: 'movie' };
+    }
+
+    return {};
+  }
+}
+
+module.exports = SABnzbdClient;