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/DownloadClient.js

@@ -0,0 +1,103 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+
+/**
+ * Abstract base class for all download clients.
+ * Defines the common interface that all download clients must implement.
+ */
+class DownloadClient {
+  /**
+   * @param {Object} instanceConfig - Configuration for this client instance
+   * @param {string} instanceConfig.id - Unique identifier for this instance
+   * @param {string} instanceConfig.name - Display name for this instance
+   * @param {string} instanceConfig.url - Base URL for the client API
+   * @param {string} [instanceConfig.apiKey] - API key for authentication (if applicable)
+   * @param {string} [instanceConfig.username] - Username for authentication (if applicable)
+   * @param {string} [instanceConfig.password] - Password for authentication (if applicable)
+   */
+  constructor(instanceConfig) {
+    if (this.constructor === DownloadClient) {
+      throw new Error('DownloadClient is an abstract class and cannot be instantiated directly');
+    }
+    
+    this.id = instanceConfig.id;
+    this.name = instanceConfig.name;
+    this.url = instanceConfig.url;
+    this.apiKey = instanceConfig.apiKey;
+    this.username = instanceConfig.username;
+    this.password = instanceConfig.password;
+  }
+
+  /**
+   * Get the client type identifier (e.g., 'qbittorrent', 'sabnzbd', 'transmission')
+   * @returns {string} The client type
+   */
+  getClientType() {
+    throw new Error('getClientType() must be implemented by subclass');
+  }
+
+  /**
+   * Get the unique instance ID
+   * @returns {string} The instance ID
+   */
+  getInstanceId() {
+    return this.id;
+  }
+
+  /**
+   * Test connection to the download client
+   * @returns {Promise<boolean>} True if connection is successful
+   */
+  async testConnection() {
+    throw new Error('testConnection() must be implemented by subclass');
+  }
+
+  /**
+   * Get active downloads from this client
+   * @returns {Promise<Array<NormalizedDownload>>} Array of normalized download objects
+   */
+  async getActiveDownloads() {
+    throw new Error('getActiveDownloads() must be implemented by subclass');
+  }
+
+  /**
+   * Optional: Get client status information
+   * @returns {Promise<Object|null>} Client status object or null if not supported
+   */
+  async getClientStatus() {
+    return null; // Default implementation - optional method
+  }
+
+  /**
+   * Normalize a download object to the standard schema
+   * @param {Object} download - Raw download object from client
+   * @returns {NormalizedDownload} Normalized download object
+   */
+  normalizeDownload(download) {
+    throw new Error('normalizeDownload() must be implemented by subclass');
+  }
+}
+
+/**
+ * @typedef {Object} NormalizedDownload
+ * @property {string} id - Client-specific unique ID
+ * @property {string} title - Download title/name
+ * @property {'usenet'|'torrent'} type - Download type
+ * @property {string} client - Client identifier ('sabnzbd', 'qbittorrent', 'transmission', etc.)
+ * @property {string} instanceId - Instance identifier
+ * @property {string} instanceName - Instance display name
+ * @property {string} status - Normalized status (Downloading, Seeding, Paused, etc.)
+ * @property {number} progress - Progress percentage (0-100)
+ * @property {number} size - Total size in bytes
+ * @property {number} downloaded - Downloaded bytes
+ * @property {number} speed - Current speed in bytes/sec
+ * @property {number|null} eta - Estimated time remaining in seconds, null if unknown
+ * @property {string|undefined} category - Download category (optional)
+ * @property {string[]|undefined} tags - Download tags (optional)
+ * @property {string|undefined} savePath - Save path (optional)
+ * @property {string|undefined} addedOn - Added timestamp (optional)
+ * @property {number|undefined} arrQueueId - Sonarr/Radarr queue ID (optional)
+ * @property {'series'|'movie'|undefined} arrType - Sonarr/Radarr type (optional)
+ * @property {any|undefined} raw - Original client response (escape hatch)
+ */
+
+module.exports = DownloadClient;