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/TransmissionClient.js

@@ -0,0 +1,181 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+const axios = require('axios');
+const DownloadClient = require('./DownloadClient');
+const { logToFile } = require('../utils/logger');
+
+class TransmissionClient extends DownloadClient {
+  constructor(instance) {
+    super(instance);
+    this.sessionId = null;
+    this.rpcUrl = `${this.url}/transmission/rpc`;
+  }
+
+  getClientType() {
+    return 'transmission';
+  }
+
+  async testConnection() {
+    try {
+      await this.makeRequest('session-get');
+      logToFile(`[Transmission:${this.name}] Connection test successful`);
+      return true;
+    } catch (error) {
+      logToFile(`[Transmission:${this.name}] Connection test failed: ${error.message}`);
+      return false;
+    }
+  }
+
+  async makeRequest(method, arguments_ = {}, config = {}) {
+    const payload = {
+      method,
+      arguments: arguments_
+    };
+
+    const headers = {
+      'Content-Type': 'application/json'
+    };
+
+    if (this.sessionId) {
+      headers['X-Transmission-Session-Id'] = this.sessionId;
+    }
+
+    try {
+      const response = await axios.post(this.rpcUrl, payload, {
+        headers,
+        ...config
+      });
+
+      if (response.data.result !== 'success') {
+        throw new Error(`Transmission RPC error: ${response.data.result}`);
+      }
+
+      return response;
+    } catch (error) {
+      // Handle session ID conflict (409 Conflict)
+      if (error.response && error.response.status === 409) {
+        const sessionId = error.response.headers['x-transmission-session-id'];
+        if (sessionId) {
+          this.sessionId = sessionId;
+          logToFile(`[Transmission:${this.name}] Updated session ID`);
+          return this.makeRequest(method, arguments_, config);
+        }
+      }
+      logToFile(`[Transmission:${this.name}] RPC request failed: ${error.message}`);
+      throw error;
+    }
+  }
+
+  async getActiveDownloads() {
+    try {
+      // Get all torrents with detailed fields
+      const response = await this.makeRequest('torrent-get', {
+        fields: [
+          'id', 'name', 'hashString', 'status', 'totalSize', 'sizeWhenDone',
+          'leftUntilDone', 'rateDownload', 'rateUpload', 'eta', 'downloadedEver',
+          'uploadedEver', 'percentDone', 'addedDate', 'doneDate', 'trackerStats',
+          'labels', 'downloadDir', 'error', 'errorString', 'peersConnected',
+          'peersGettingFromUs', 'peersSendingToUs', 'queuePosition'
+        ]
+      });
+
+      const torrents = response.data.arguments.torrents || [];
+      logToFile(`[Transmission:${this.name}] Retrieved ${torrents.length} torrents`);
+      
+      return torrents.map(torrent => this.normalizeDownload(torrent));
+    } catch (error) {
+      logToFile(`[Transmission:${this.name}] Error fetching torrents: ${error.message}`);
+      return [];
+    }
+  }
+
+  async getClientStatus() {
+    try {
+      const response = await this.makeRequest('session-get');
+      const sessionStats = await this.makeRequest('session-stats');
+      
+      return {
+        session: response.data.arguments,
+        stats: sessionStats.data.arguments
+      };
+    } catch (error) {
+      logToFile(`[Transmission:${this.name}] Error getting client status: ${error.message}`);
+      return null;
+    }
+  }
+
+  normalizeDownload(torrent) {
+    // Map Transmission status codes to normalized status
+    const statusMap = {
+      0: 'Stopped',      // TORRENT_STOPPED
+      1: 'Queued',       // TORRENT_CHECK_WAIT
+      2: 'Checking',     // TORRENT_CHECK
+      3: 'Queued',       // TORRENT_DOWNLOAD_WAIT
+      4: 'Downloading',  // TORRENT_DOWNLOAD
+      5: 'Queued',       // TORRENT_SEED_WAIT
+      6: 'Seeding',      // TORRENT_SEED
+      7: 'Unknown'       // TORRENT_IS_CHECKING (alias for 2)
+    };
+
+    const status = statusMap[torrent.status] || 'Unknown';
+
+    // Calculate progress and sizes
+    const progress = torrent.percentDone * 100;
+    const size = torrent.totalSize;
+    const downloaded = torrent.sizeWhenDone - torrent.leftUntilDone;
+
+    // Handle ETA - Transmission uses -1 for unknown, -2 for infinite
+    let eta = null;
+    if (torrent.eta >= 0) {
+      eta = torrent.eta;
+    }
+
+    // Extract category/labels
+    const labels = torrent.labels || [];
+    const category = labels.length > 0 ? labels[0] : undefined;
+
+    // Try to extract Sonarr/Radarr info from name
+    const arrInfo = this.extractArrInfo(torrent.name);
+
+    return {
+      id: torrent.hashString,
+      title: torrent.name,
+      type: 'torrent',
+      client: 'transmission',
+      instanceId: this.id,
+      instanceName: this.name,
+      status: status,
+      progress: Math.round(progress),
+      size: size,
+      downloaded: downloaded,
+      speed: torrent.rateDownload,
+      eta: eta,
+      category: category,
+      tags: labels,
+      savePath: torrent.downloadDir || undefined,
+      addedOn: torrent.addedDate ? new Date(torrent.addedDate * 1000).toISOString() : undefined,
+      arrQueueId: arrInfo.queueId,
+      arrType: arrInfo.type,
+      raw: torrent
+    };
+  }
+
+  extractArrInfo(filename) {
+    // Similar to SABnzbdClient, try to extract Sonarr/Radarr info
+    
+    // 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 = TransmissionClient;